Some of the best pre amps used for $600 or less

older CODA linestage

CJ PV10a

Perhaps a Dehavilland? They are underrated and underpriced!

I second the McCormack idea: I think you can get an ALD-1 for around that price. If you want to upgrade later, another $1300 through SMC Audio takes it to a level that few preamps under $10k can match.

good luck with your search!

Forgot to add the Eastern Electric Minimax. I sold mine for $590, and it was a great preamp, sounded awesome in my system.