Is "used" a brand? A best-buy would be something like a Crown XLS 1500 or higher...I run a stack of 'em in the PA and have used both the 1500 and 2500 with (2) Acme B2's. My rack now has a PLX 1602 which came from lord-knows-where. You can run one enclosure per side or daisy-chain (creates a 4 ohm load) and run the power amp in bridged mono.
Riis
I've thought a lot about one of the Crown Drivecore amps. The XLS1500 would be enough for two 8 ohm Low B2s?