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

I'm sorry that I cannot comment on tubed gear, but can give you solid recommendations on Solid state.
Audio Research LS3
Classe 5(I don't think you will find a CP5 at this price)
Muse Model 3 (wonderful piece of gear)
Bel Canto Pre 1 ( I have seen $700.00)
Adcom GFP-750
There are more, but these come off the top of my head. These are all incredible values and you can't loose for detail and resolution on any of these. They do vary quite a bit for how they present music, but are all top notch performers.

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.