Best Class A Amp under $2,000 used?

You might start with Audio Bluebook on this site.  See what's in the market.   https://bluebook.audiogon.com
The triodes (valves) plus Class A power supply appears preconception more than a criteria which assures a quality experience. 
Unreasonable constraints elevating legacy technology (& sub-optimizations) works against you, not for you.  There is a point in the used market where you are just buying someone else out of their boat anchor.  This is business built on scale.  As the components, and various engineering trade-offs, of legacy systems become rare- available inventory inelasticity changes the value...non-linearly.   This all works against you.  Still- "may the odds be ever in your favor", though. 

Something closer to a Pareto curve.  Its a surface, not a point.  Amplifiers are systems-of-systems.  Transformer technology has evolved.  Discrete components are no longer +/- 5%.  Some are now chip-ed assemblies rather than PC boards. And line widths are under 10 nm with the upper end of 80 nm dissipating from stocks.  Speakers have changed. Test equipment has evolved.  Recording and mixing technology changed (evolved, arguably).  You have devices now that clock in nano (soon to femto) and a few 10s of milliseconds vs micro and hundreds of milliseconds. RIAA curve has evolved. FCC and CISPR measures and isolation have changed. There are some who find comfort in what they once knew. Economic market value isn't just set by taxonomy- alone or in large part.  Authenticity is now argued with newer, non-deterministic math. Still, some cling to subjectives in prose and poetry to evoke a consensus. Market valuations are just a sum of demand against a supply.  Baby boomers will still prefer a signature sound they have trained themselves listen for. Greenspan's 'irrational exuberance'.