With 400W RMS, IMO it's better to get a single sub that is better and more powerful. Then you can spend $300 bucks on a much better subwoofer, such as the CDT HD-1020 or the JLW6v2. Both are around $300 online and can be found at www.woofersetc.com, and are fantastic subwoofers. If you want SQ/SPL, try a ported box, tuned lower such as around 30-32 hz, it'll give a deep, powerful response and retain great SQ.
Ok... if you are suggesting only pushing 1 sub with this amp. would a single 12" be better? Perhaps the eclipse aluminum?? Would this 400w RMS drive that sub ok? BTW, what do you think about the MTX amps.. it is a MXA4001. I know it's nothing great, but in that class, is it an OK amp?