At $500 for a tube unit, I'd think more about condition than sonics.
Also, with your amplifiers rather low single ended input impedance, it might narrow your most likely to be single ended output tube preamp options. Not sure if you need a phono stage, that'll probably add to the cost.
Here's a quick off the top of my head shortlist of inexpensive tube pres,
caveat: don't know it they'll work with your amp, or if any fit any of your criterion, but at $500 you're not going to get too many options:
Cary SLP 50
Conrad-Johnson 10
EAR 834
Quicksilver Line Stage
VTL TL-2
I'm sure there's more,
good luck!
