Preamp recommendations for under $600.00

Audio Research LS1

Counterpoint 7.1

Forte Model Two

Audible Illusions 2D

Parasound P3

conrad johnson PV7 or PV6

Classe DR4 (Four)

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.