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

Conrad johnson pv 10, Rogue 66

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.

The McCormack TLC-1 regular or Deluxe are great preamps for under $600. For tube preamps I've read that a Bottlehead Foreplay is a great sounding one as well. You see them pop up every now and then here for under $600.

Bottle Head.

The Dared sl-2000a beat out my old Rogue 66 mag and my buddies VTL 5.5