Subs are evaluated in the same way as any other speaker. Look for the best bass extension, with the most output at that frequency, and smooth response from that point up to the crossover frequency. Be realistic too - you aren't going to get solid 20Hz response at any useful output level from a cheap subwoofer that uses an 8" driver.

The amplifiers used in subs are well matched to their drivers, so I wouldn't let the little details about amplifier class or buzzwords sway you one way or the other.

Multiple subs provide more output and because of mutual coupling between them, have a better low frequency extension than one of the subs alone, and can deal with room problems in some cases.

You could spend all the money in the world on a sub, but if you have a problem room, all that money is wasted. In a situation such as this, two lesser cost subs might be better than one super expensive one. You must evaluate your room first to see if there is the potential of strong room nodes causing problems, such if you have a perfectly square room.

------------------
The Soundhound Theater