It depends on the amp, of course, but many single ended amps and pushpull amps using just a pair of tubes like 6L6 or KT-66 deliver a full, lush sound without sounding muddy or sluggish. Within their modest power levels, they are quick and dynamic and the sound is "dense"--harmonics are complete and realistic and not lean and thin. I don't like most high powered tube amps with multiple KT88, KT120, KT150 because they tend to have a hard and brittle sound--the initial attack is artificially edgy and not as natural sounding. Most high powered solid state amps sound lifeless to me at modest volume levels. There is also an artificial sounding edginess to the attack of notes that makes them a little bit brittle sounding or as some people put it, there is a "glassy" sheen that seems to be always present.. A good tube amp sounds relaxed and not edgy, yet when the music calls for it, they sound lively and deliver dynamics without sounding hard.
A decent 40 watt amp (e.g., Synthesis A40) should work with the majority of speakers, but, some panel speakers might require a bit more, depending on the setting and circumstances. I don't rule out ANY speaker type, and certainly there are many that I enjoy even when driven by amps that are not my personal ideal amps. It is just a part of juggling different strengths and weaknesses and making the right compromises. I could easily live with something like the 30.7 Magnepans and an Ayre amp, although I still prefer a good horn system and my 5 watt tube amp.
