Your initial premise of cleaner sound from a small amp at low levels might have a grain of truth, but for all sorts of reasons from economics to designer ego, small solid state amps are rarely an engineering tour de force. The emphasis is on low cost, which means design shortcuts or at least simpler topologies that don't usually perform as well. One certainly can build a very good low power amp, but the market would be headphone amps or the low power tube amps you've already got. I'd ignore the power limitation as artificial and just look for the cleanest integrated. Exactly what that would be is certainly wide open and subject to taste. One man's clean is another man's harsh.