The very best sound quality is from single-ended amps.
@imjerrys They don't really make proper bass- most SETs that make 7 Watts or more can't make full power at 20Hz. The output transformer simply lacks the inductance to allow it. If it did have the inductance it would be much larger and then wouldn't be able to make the highs.
Distortion is quite high, which is part of the lush quality, but that distortion also obscures detail.
You can have amps that are much lower distortion so are more detailed, but every bit as smooth, while also making proper bass. So SETs can be fun, but in no way are they the 'best'.