that depends on your budget. large high quality mono's can be pricy (ok, are pricey). my dedicated 2 channel rig, doubles as a theater, or vice versa.
The Boulder 2050's are one of the best SS mono's available (IMO, the best). I prefer their sound to the large Halcro's (i didn't think it was close). The Boulder 1050's shouldn't be far behind (i have only ever listened to the 1060 stereo version). Classe CAM350's, MAC501's, 401's, 301's (not sure which of MAC products are still current). I have never listened to the Pass amps, but they seem to be well thought of in the various press/forums. Even the high end Parasound Halo JC-1's have been well received (but i haven't heard them). I have never demo'ed the Krell equipment. I really liked the ML 33's, but that was too long ago to be a good data point. I like what I have heard of the Theta Digital mono's (and Dreadnaught) as well. That's an "all over the map" list.
It is important to have "balance" in the system. If the amps are significantly superior to all else, you may not hear all of the dollars you spent on them. Of course, you could slowly upgrade everything else in the system over time as well. I'm kind of proponent of really blowing the budget once, getting what you want/need/can afford, and being done with it for a while.
Amps that run is pure class A mode, will in general, sound better than A-AB amps, but will throw off lots more heat. Pure Class A amps only make sense IMO if the system will also be a 2 channel setup for critical listening. If not, then I would lean more towards a less expensive A-AB amp, and save on the cooling budget. Depends on where you live and whether AC is a way of life, or an added expense. In the northeast USA, pure Class A amps are not a problem, as the heating season and hvac neutral season is far longer than the usually short (2 month) cooling season (when we are outside playing anyway).
