Biggest disappointment: Carvin R1000. Just couldn't get a sound I liked from it. Second biggest disappointment: my first "big boy" amp, an Earth (model no. escapes me). Black vinyl tucked and pleated exterior (it WAS the 70's). Bought it used from "some dude". Some days it sounded good, other times, seemingly due to its own will, it would start whooshing like some phase shifter from hell and the power would drop to barely audible. I spent a lot of money I didn't really have trying to get it fixed. Finally sold it as a package with my '77 Rick 4001 (I wish I had THATback now).
Exceeded expectations, because they were appropriately low: GK Backline 110. All I wanted was a practice amp that didn't scare the crap out the dogs or annoy the neighbors. I had taken a sabbatical from playing for 20 years and just wanted to get that back in my life. What I got for my $100 was a decent sounding practice amp. I actually bought the Carvin after this and was sad that it sounded so poorly, since I love my Carvin LB75 bass.
I'm looking for my next "big" amp to power my Avatar 410n cabinet-- another "exceeds expectations" purchase. I have been considering a Genz Benz GBE 750, a Shuttle Max 9.0, a Little Mark Tube 800, a GK 1001RB, and a Trace Elliot AH600-12. This thread has been a big help.