Preamp recommendations for under $600.00

My 2 favorite in that ballpark range are:

Musical Fidelity A3cr
Audio Mirror T61

preamps require care in system matching, so one that sounds outstanding with a specific amp and speaker may sound dreadful in your room. i've been through a few - counterpoint, aesthetix, etc - and am currently using a placette passive. given the appropriate gear around it, it's an absolutely stellar unit. depending on the number of sources you've got, the placette RVC may work and can be found used for a little over your stated budget.

I'm not sure how it matches up against the above suggestions but I had a Marsh preamp about 10 years ago that performed very well for the money. It did come with a remote which you need to keep in mind when considering older models.
Marsh had a good phono section too, as I recall.

If you don't need a remote and don't mind an older preamp in the $600 or under price range my favorites are the McCormack TLC-1 Passive Preamp and an Accuphase C200.

If you need a remote a favorite of mine is the Kavent S-33 preamp. It also goes by the Vincent Audio SA-93.

Tube wise I've read good things about the Dared SL2000A.

To get better help let members here know what other components you have in your system or other gear you have enjoyed in the past. Also letting members know what you feel is lacking with your current system will help with recommendations as well.

At that price, I would use a passive unit or a source that has a volume control.