First, that mechanical sound is probably the subwoofer's coil bouncing off the back plate. This is very bad for the woofer and you're likely damaging the subs. Good news is you can recone the woofers(which means you replace all the softparts) unlike other brands. The reason you're damaging the subwoofer is likely due to using that enclosure and playing the subwoofer below tuning without a subsonic filter. when playing a subwoofer is playing below tuning the sub can "unload" which means the cone will travel beyond the mechanical limits (move too far in and out). When you're using a ported enclosure you must set a subsonic filter just below the enclosure's tuning frequency. This filters out the signal below that point. The enclosure, like most pre fabs, is probably tuned rather high and you lack of subsonic filter allowed the sub to damage itself. The first thing you should do is figure out the tuning frequency of the enclosure and set the subsonic filter properly. If you're happy with that than you're set. But its likely that the enclosure will perform like shit and you should build a proper one. also, assuming you wired the woofers up correctly to 2 ohms. it's impossible to wire them to 1 ohm. but that difference in power wont be noticeable so dont worry about it. Sorry about the amp model. Its a HFI1500D and it does have a subsonic filter. 15 to 35 hz. Also its only done it a few times. I skip the songs that do it. I race bikes and cars so if Spotify plays a new song and it starts unloading I have a good reaction time. I don't want to damage them. I have good mechanical skills and I'm an electronic technitian. I also turned down the bass on the HU and amp.