The BTL line is our flagship woofer. Its whole design is based on and focused on being able to throw tons of power to something in a Bass Race or �Street Jam� situation. Options Available are as follows: P Chamfer: This is a chamfer on the end of the pole, it helps to bring the air down around inside of the motor and helps cool the coil better. It also helps relieve the pressure underneath the dustcap. We recommend at the bare minimum to get this option if you choose this series of woofers. Cooling: Self explanatory, the cooling channels are machined into the inside of the sub to help cool the coil FAR better. But there are some things that you sacrifice. There is less metal inside of the motor so it is a little weaker. If you are looking for a fart/burp only scenario for SPL we suggest that you do NOT get this option. Stick with only the P chamfer. Now if you are looking for a daily sub to beat on and you have 2000+ watts the BTL series is for you. Flatwind: The flatwire coil option is standard in the BTL. HiXmCoil: This coil is simply much longer than the normal dual 1ohm or dual 2ohm flatwire coils and allows for more xmax. If you are using this in a daily scenario we suggest that you choose this option, especially in the 15 and 18 inch models. Inner Heat Sink: Its a machined aluminum ring in the gap that lowers the voicecoils inductance (allowing it to play up higher and have a little better overall transient response). It also reduces the volume inside the magnet gap of the motor increasing air speed with the cooling as well as allowing the coil to kick off heat to a more receptive thermal mass rather than magnets. Daily or SPL: Simply put, if you plan on listening to any type of music you MUST choose the daily option. If it is strictly for a fart/burp scenario then choose the SPL option. The reason we have these options is the gap tolerances between the two are different. The gap in the SPL woofer is much tighter and FAR less forgiving in comparison to the Daily option. The Daily option simply gives the coil more wiggle room inside of the gap to deal with the non-linearity tendancys that you can get into when you are using a sub in a daily driving scenario. Typically SPL applications deal with 1 note and it does not change, therefore we can make the gap much tighter and get all of the motor strength that we can out of it. Universal: This option gives you the choice to later chance the impedence of your voice coils. You can change from dual 2�s to dual 1�s and vice versa by reconing them. If you do not choose this option your motor(s) will be optimized per the coil resistance that you have chosen. (Dual 2�s will forever be a dual 2 motor, Dual 1 will forever be a Dual 1 motor). Enclosures: BTL 12 � Ported box we recommend using 1.75-2.5 cubic feet @ 30-35Hz (I personally think that a 32hz tuning is optimal, and has the most output and is still the most musical daily driving). Use 12-16 square inches of vent area per cubic foot of volume. BTL 15 � Ported box we recommend using 3-5 cubic feet @ 30-35Hz. Again use 12-16 square inches of vent area per cubic foot of volume. BTL 18 � Ported box we recommend using 5-8 cubic feet @ 30-35Hz tuning, you can tune a little higher if you wish but we recommend keeping it close to 32hz for a daily driver. Again the same 12-16 square inches of area applies per cubic foot of volume. Pictures and cut sheets for some enclosures coming soon.