Poor Bass response

HI sevenprunes, first try disconnecting all woofers then reconnecting one woofer at a time to verify that each is physically in phase. Just because they are electrically in phase doesn't guarantee that they are physically in phase. Also check your crossover point. It may be that it's set too low or too high. Yes, the volume of the car does affect the sound but shouldn't make it as bad as you described unless the remaining volume is extremely low. Placement is another possibility, but you didn't tell us where and how you mounted your enclosures.

Let us know how you make out.

the12volt

One other thing worth a shot if all else has failed.... you said the amplifiers see a 2 ohm load. While the amplifiers may be very stable at 2 ohms, it may sound much better if they wired to see a 4 or 8 ohm load. Often running an amplifier at a lower impedance will dramatically diminish the amplifier's performance and overall sound quality.

the12volt

I would have a look at crossover points...often, the real kick of a bass hit is as high as 200 hz. A good sub may sound like crap that high. If the rest of your system (Esp. Midbass). If your running the mids/highs a lot lower than your subs, it can almost make it sound like it has no punch.

One other thing - It may behoove you to try a subsonic filter (Maybe onlyn on 2 of the 4). It could be that hitting the extreme low fequencies (18 hz is really, really low) is washing out the punch. To hit a sound that low, the cone has to move pretty far and pretty  slowly (relative to other speakers). It may be that the cone can't move fast enough at the attack to make a good punch. Maybe try crossing two subs at 0-(whatever you're at now) and two at 20-180 (tune the upper point by ear). If you have a lot of music with bass down below 20 hz, the cones may be doing a whole lot of movement to produce, say, 12 hz, where you can't even hear it. If you cut those, it could give you seriously better response (dB wise and punch wise).

/NyxBass