On the low end, any Bose Acoustimass I have ever heard sounds on par with most higher end computer-audio speakers in the $250 range.
On the high end, Wilson Maxx (the new ones, whatever number they have) driven by Halcro/Meridian left me cold. Unbalanced, thin, dispersed, totally unacceptable for a $150k system.
Then again, on one visit to Denver I heard some of those huge Focal JMLab $85k beasts driven by Boulder amps and it was also crap sound quality for the $200k that dealer was asking.
I've heard plenty of damn good systems in the $50k range, for some reason the sound quality seems to start deteriorating when you go much above that!
I'd also give a dishonorable mention for Musical Fidelity's amps from around five years ago - they could make any speaker sound harsh, brittle, and overly analytical. I haven't bothered to listen to them since.