We do a lot of in-floor subs and prefer that method to any other out there. The in-floor approach gives us the ability to scope a room and build the enclosure to work with the room acoustics and avoid the "generic black box". Under the floor allows for the most flexibility in design and tuning. A good bandpass enclosure can have a volume of over 6 cubic feet when tuned to a large room and a vent that is over 30" long. We have also found that the mounting of the subwoofer enclosure to the subflooring from underneath gives the highest level of low frequency transfer. the cosmetics of a matching floor register also means the sub is all but invisible unless you know it's there. The only drawbacks to this apporoach that we have found are the level of technical skill required to properly design and tune an enclosure (there is good software out there, but it still requires a higher than average understanding of room acoustics and some wookworking skill) and making sure that you can physically get a 14" x 14" x 42" box under the house and to the location specced during pre-wire. As for amplification, the amplifier is matched to the driver and the power requirements of the subwoofer, and the wired speaker level. This also helps to avoid noise that can be induced over in wall runs of low level signal(just because you avoided the electricians' wire, doesn't mean they stayed away from your signal wire after you left). We custom build roughly 75 percent of the subs that we install, even the ported, or sealed ones in cabinets. I understand you have a concrete floor issue so this approach is probably out of the question, but may be some food for thought for the next install.