Just to provide the counter-point...
There is something to be said for the amp being specifically matched to the sub!
I would be absolutely on board with these being external amps, but most of the major ID players have done a more than casual job of matching the amp to the sub/driver.
One of the most important attributes of a sub is what it does when driven to it's limits - it can push the driver to the point of mechanical interference (a nasty sound to invade your audio bliss), or to the point of port chuffing, or worst case, blow the driver (especially likely at frequencies below the port tune of a ported sub)!
Tom V. of PSA will tell you not to take it as a challenge, but his subs are pretty-much bullet proof! I think that is pretty much true of all of the major players. Ideally an amp will employ a limiter before the threshold where any of the above problems begin. Thus when the signal would create the problem situation, the sub simply does not play at the full level that the signal suggests. Many times, you would never recognize the condition because it is handled so adroitly.
Again, I would be totally on-board with these "specific to the sub" amps being external, but I do want that type of attention to the mate up.
Every DIY sub builder has to deal with this problem in one way or another.
I think the best solution, assuming you have the budget, is to select a sub that is so potent that you would never see a situation where the sub is getting to its limits! It sounds like the RBH sub under discussion qualifies on that count, but I think it is important to discuss the disadvantage of external amps for the benefit of readers that have not considered the drawback of using external (third party) amps with a subwoofer and may be looking at less expensive options!
