Sound quality is very open for interpretation in this context - a flat guitar amp has very dissappointing sound quality for a guitar setup as particular kinds of distortion are wanted and others are not (in particular very soft clipping is essential and frankly only valves or emulating them digitally does a good job at this).
Similarly a distorting general purpose amp has very dissapointing sound quality, and are expected not to be driven into clipping constantly.
It depends which qualities matter in the context, and the context is a guitar amplifier. Well that's how I read it - and you conflated this with another issue of audiophoolery, leading to my misunderstanding.