hifi engine

Best Used Power Amp for $550 or less

I am going to be buying a power amp very shortly and I need some input.
I am looking at a Audio Research D100, crown psa2 and K1/K2 but I am sure there are a wealth of other products that I don't know enough about , so I would welcome your input.

Some products I currently own and like
Crown PSA 2
Crown Powerline 4
Emotiva 7.1 AV preamp
DCM Timewindows
Snell IV type C
Wharfdale EVO DFS surrounds (these are amazing!)