Best Class A Amp under $2,000 used?

For 250 watts of pure class A, a pair of Threshold SA/12e’s would definitely work, however if you find a pair, they would be well above $2000. Here’s a nice Threshold within your budget,

http://www.usaudiomart.com/details/649302974-threshold-s350-power-amplifier/

This amp could also be changed to pure class A, but only at 60w/ch. You would need to send it back to Jon Soderberg of Vintage Amp Repair.

That Threshold "e" series was the last Nelson Pass design at Threshold. Also, the recent work on the amp was done by Jon Soderberg. He worked for Nelson Pass at Threshold, so there is no better Threshold tech anywhere.