Generally, lower power amps (around 20 watts) sound better than higher powered amps at low volume levels. Most of the time these amps are tubed and sound "alive" at low volume levels unlike solid state amps which tend to go dead. The only exception to this that I know of is the 47 Labs Gaincard, although a pure class A (Pass Labs) may have this characteristic.
Also, as pointed out in the above post, speakers are very important. Electrostatics have excellent low level resolution as do hybrids (ribbon with dynamic cones). Speaker design is also very important, but it seems that bigger speakers do not resolve as well at low volumes as do their smaller driver counterparts. Driver size may have something to do with this.