I've got a couple of Quad 405-2's waiting to be modded a la Keith Snook http://www.keith-snook.info/quad-405-mods.html. The subs have two drivers in each of them. Each driver will be driven by one channel of one amp. Since the subs have a 6dB/octave roll-off (approximately) I need a factor of 4 power increase per octave from the amp. If I cross the subs over at, say, 120Hz and the required amp power there is, say, 15W/sub, then I'll need 60W/sub at 60Hz and 240W/sub at 30Hz. A modded 405-2 should, just about, deliver 240W into 8ohm loads.
Of course, modding the amps is yet another job. As is sourcing a crossover (almost certainly DSP) to sort out the feeds to the various amps.