I know that it is a gross oversimplification, but, as long as one has sufficient power, lower powered amps tend to sound better than behemoths. Also, some higher powered amps sound particularly dull and lifeless if they are made to loaf along at really low output.
Geoch has offered some really good choices for getting very lively sound at lower volume level. I like OTL amps, though I am more familiar with the smaller Atmasphere amps (the 30 and 60 watt versions) driving speakers with more modest requirements. I have a Placette Active linestage and it, too, is particularly good at sounding very lively (moreso than the Placette passive, and more than my other solidstate preamp, which is a Levinson No. 32).
As I mentioned earlier, I too like electrostatic speakers for low level listening. Not only do electrostatics deliver detail without being strident, the dipole bass cancellation effect to the sides means less bleeding of sound into other rooms (so one can play them a bit louder). For conventional dynamic speakers, I think Audionote floorstanding speakers and stand-mounted speakers are also very good at low levels. I've also heard small DeVore speakers, powered by Shindo amps, that sounded very good at lower volume levels.
Mapman mentioned Triangle speakers. I also think these are terrific low level speakers. Some people find them a touch bright (they are), but they are not strident and are good candidates for low-level listening.
I have also listened briefly to some Living Voice floorstanding speakers that sounded quite full and naturally balanced at lower volume.
For late night listening at more than modest levels, I use headphones. Sure, they don't deliver a conventional soundstage, but they can actually be more engaging and have MORE visceral impact (than listening to speakers at low level). I have gotten used to the inside-the-head sound and so I appreciate them as a completely different experience. The speed and freedom from resonance (from either the drivers or the room) muddying up the sound is a big plus with headphones. The headphones I use are Stax Omega II, Mk. 1 with HeadAmp Blue Hawaii Special Edition headphone amplifier.