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

using a PS audio HCA-2 amp and Revel F 30 speakers
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
Absolutely try the Audio Research LS-3.
I remember driving 50 miles to meet a seller at a mall to buy a Dared SL-2000A. I think I paid $300.00 and it came with extra tubes for tube rolling. When I got it home and hooked it up, I wasn't expecting alot. But boy was I surprised by its sonics. Huge and tall soundstage from such a small preamp. Big surprises do sometimes come in small packages & I agree with David99's conclusions. One can say this is an excellent performer at its price point but it's much more than that and could easily be compared to alot more expensive preamps, you may find yourself smiling all the way to the bank.

Another Chinese tube preamp that offers alot of value and packs a mean but excellent sonic punch is the new Norh ACA-2B. Sells for $299.00 plus shipping. Shipping however is about $175.00. You can't go wrong with either preamp.
AES DJH (made by the folks at Cary) is currently available for $699 on Audiogon (No, It's not mine). While I have since "upgraded" to much more expensive preamps the AES DJH is a wonderfully musical tubed linestage at a very good value.
The Doge 8 lists at around $1300. It is a great pre-amp. If you can find it used, gobble it up.
Throw the Quicksilver Audio tubed linestage into the mix, can be had used for about 600. I almost purchased the dared as well, if I saw one come up used for 300 I would definitely jump on it.
I second the Doge 8. Sorry now I sold mine now. Not sure you can get one for under $1000.00 used though.
You might want to look into the Mapletree Audio linestage, two box, tube regulation, variable gain, and $760 new! Made by hand, point-to-point by a retired Canadian electrical engineering professor and audiophile.
There is allot of Krell stuff that SHOULD BE 600, but people keep overpriceing this stuff. It is great if one can get it cheap enough and when it breaks you don't really need to worry- just get another one if it is cheap enough.
musical fidelity a3cr, hard to find cause they usually sell quick at $600 or under
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.
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.