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

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

I never thought of the DeHavillands as underated - underpriced? perhaps - but I think they are pretty highly regarded, certainly by those that have heard them.

You may want to try tubes to pair with your PS audio amp. The EE minimax mentioned earlier is pretty universally praised. You could also sell it off for little if any loss, I would think.

Every preamp on here is a winner in its own right and as stated earlier, not on any one piece, but most of these you could resell at a minimum loss if any. I suggest to buy one and start the hunt.