Impious

Could be several things. Could be those songs are heavily compressed and have little dynamic range, so as you increase the volume you are clipping the amps (inputs, not just outputs) or running into power compression issues on the drivers.

Using a DMM is not the best way to set the gain. Neither is using an o-scope. Nor the DD-1. Simplest and best solution is to set it by ear.

I don't think it's possible to describe beaming in subjective "sounds like" terms. It's also going to "sound" different depending on the severity of the beaming (i.e. how far off-axis you are from the speaker) and the frequency range over which it's occurring (midrange, treble, etc). Every speaker will begin beaming at some point, from subwoofers to tweeters. Beaming is basically a reduction in the amplitude of high frequency off-axis response of the speaker compared to the on-axis frequency response. If you were walking around a single speaker playing full range you would notice the high frequencies start to taper off as you walked from on-axis to off-axis. The higher frequencies wouldn't be as loud off-axis compared to on-axis. Where the driver begins to beam depends on the diameter of the speaker. A larger speaker will begin to beam lower in frequency than a smaller speaker. Beaming can create several potential problems....undesirable frequency response at the listening position from the direct sound from the speaker due to the "dip" or "hole" created by the beaming, uneven frequency response between the left-side and right-side speakers since you generally don't sit at the same degrees off-axis from both speakers, and larger deviations in direct vs reflected sound at the listening position. All of this cause issues with frequency response and imaging causing things to simply sound wrong. You might not be able to point a finger at it and say "Ah ha, that sounds like beaming!". But if the problem is in the midrange (for example) then the midrange frequencies might sound wrong or off compared to the rest of the bandwidth and the imaging will be skewed or scattered, for example. FWIW, I really like Andy Wehmeyer's philosophy behind designing a car audio system. He didn't invent the idea, but he's one of the bigger proponents and has spent a significant amount of time explaining it from an acoustics stand point. Pretty simple really. Cross all of the drivers within their piston range to avoid beaming, wave guide the tweeter (since you can't avoid beaming) to create a smooth on- to off-axis transistion, and don't mount the driver's right next to reflective surfaces (i.e. avoid A-pillar or top of the dash installations). Certainly it's possible to break the rules and create a system that sounds subjectively good without taking all of this advice....but it provides a good foundation to start from if you keep these things in mind.

First, the subwoofers don't "need" any particular amount of power. It doesn't matter if the amplifier provides 500w per sub or 400w per sub, you won't hear a difference. As far as the amps....you again won't hear a difference between them. Go with which ever is cheaper, which I'm guessing will be the PPI. The PPI's are solid amps, do rated power and there are a dozen different brands that use those same boards so they are relatively proven.

Never ask for or take any advice from that gentleman again. And yes, there are a lot of SPL type guys who put speakers all over the place, including in the middle of the car, and think it sounds "great". They are wrong....it sounds terrible, they just don't know enough to know it.

Waste of money and terrible idea. Stereo sound works because you have left and right speakers. Having speakers in the center of the car, without proper surround sound processing making them dedicated center channel speakers, will absolutely destroy the stereo sound.

IMO look at Zed's user manual and draw some inspiration. His may be a little overboard, but it goes well beyond just explaining the product and includes quite a bit of educational information. http://zedaudiocorp.com/pdfs/ZedManual-2012.pdf

You are not understanding. The speakers are 4ohm. You don't change that. You can't change that. For the 2-channel amplifier you wire the speakers TOGETHER in parallel (both left on one channel, both right on the other channel), and the amp will "see" a 2ohm load because there are two 4ohm speakers in parallel and that presents a 2ohm load to the amplifier. As long as the amplifier is 2ohm stable, then nothing bad will happen. Each speaker will receive half of the power. So if the amp does 160w @ 2ohm, each speaker would receive 80w (I have no idea how much your amplifier outputs @ 2ohm, don't have time to look it up).

This is your idea of a driver designed to excel in small sealed enclosures?

What speakers do you have? What amplifier do you have? If the amplifier is 2ohm stereo stable then you could simply wire them in series to the amplifier. You would lose front/rear fade, which you would retain if you went with a 4-channel amp instead. Depends on your goals and budget.

I drove by your house yesterday

No, you'll be down quite a bit of output over the main subwoofer passband. What vehicle are you installing in ?

Assuming the setting is an actual level setting for the subwoofer output on the headunit and not a bass boost, then there is zero difference between a low set gain and a higher level setting on the HU and a higher set gain with a lower level setting on the HU. Sounds like you might have simply had the gain set too low to start with.

There is zero need for this product in the market. That's not innovation.

Downsides to common chamber are the lack of bracing provided by the separator (not an issues as long as you use adequate bracing elsewhere), and if one driver fails it potentially increases the ability to mechanically damage the remaining good driver due to the change in alignment. Otherwise no issues.

Efficiency doesn't correlate to output capabilities either. Don't you mean sensitivity/SPL rating (1w/m or 2.83V/m). Efficiency does correlate to output, because efficiency=(Usefull energy/Total energy)*100% . (which in this case is acoustic energy divided by energy put in the driver times 100%) I was referring to reference efficiency, which is the percentage of electrical input converted to acoustic output. Sensitivity is simply reference efficiency expressed in the decibel scale. And it doesn't mean anything as to which driver will ultimately have higher output capabilities.

I believe you hit the nail on the head. The goal is to limit ground loop noise than actually isolate noise during the sound transmission. As an aside - My old Coustic DR amps accepted balanced input via dual RCA inputs per channel.

Question everything that isn't accompanied by suitable evidence. This applies to most things in life, not just audio. I had a 5 page argument on another forum years ago because it was "common knowledge" that Sony amps were over rated. Yet when I presented 3 different tests of 3 different amplifiers from 3 different sources and every single one of them met rated power, it was still disputed because of the "common knowledge".

Efficiency doesn't correlate to output capabilities either.

The original PDX amps were confirmed by werewolf to have differential inputs per the service manual back in the ECA days. The designers at JL and JBL are pretty smart cookies....from the posts from Manville and Andy, respectively, I would readily believe the amps they indicate to have differential inputs truly do. Seems like of all of the things to falsely advertise, differential inputs would be a bad choice given the first time someone tries to drive it with a balanced signal and the amp goes pop, there would be a lot of liability there. From what I found skimming threads a while back some BMW systems use balanced line level signal transmission and people have been splicing in directly to the OEM system by using amps with differential inputs.

And it's a load of bullshit. With very few exceptions, every single time someone says that it's bullshit. You're simply gullible if you believe it. Ask for PROOF, and those people can never provide it. Because it's "common knowledge", and they heard that "common knowledge" from someone else who couldn't provide a lick of evidence and were stupid enough to believe it. The actual measurements I've seen of the newer SS amps have all met rated power. I see no reason to expect otherwise. Good. Wasted money isn't good. I'm assuming you are running the SS amp at 2ohm per channel or bridged into 4ohm giving your subs right around 300w a piece, there realistically isn't a power "upgrade" from that without changing subwoofers since you are right at the driver's rated power handling. Increasing power enough to be audible isn't going to be beneficial as by that point you will be far exceeding the thermal limitations of the driver.

Higher rated power handling doesn't mean what you are attempting to infer from it. SSD 18 in grey Fi BL 18 in yellow SSA Icon 18 in blue AA Chaos 18 in red All in 6cuft 33hz w/ 1200w. Power handling on all of them is reasonably close enough I don't really foresee significant differences in power compression. Point is...they are all going to perform very close to the same audibly. They are all within 2db over the entire bandwidth, with the exception of the BL due to the saddle in the response.

Because the sounstream amp probably puts out 200-250, Id rather have more then I need and still get the same sound so that I don't risk clipping like I am now. I am planning a new build but id rather have everything right in the mean time, before I start buying stuff for the new build. That was my logic at least, and honestly I like trying new things and seeing how they work out. Again, you doing nothing but guessing. What facts do you have to support either premise that the Soundstream is overrated or that the RF is underrated? Clipping has to do with your settings, not the amplifier. Sounds like you're trying to throw money at a problem that exists in your mind, not in your system. Spend the money where it will make a difference, and right now I have no reason to believe that is the logic you are following.

I would think that wouldn't be true though.... If I was switching from an SSD 18" to a BTL 18", even on the same power it should be louder. Why? What in particular makes you say that?

I assume you are reusing your current enclosure? If so, what is it's net volume & tuning? Performance depends on the enclosure. You can't just ask "if the enclosure was the same.....", still need to know what that specific enclosure is.