Sleeper preamps: $500 new/used

Prob get a Jolida Tube Preamp for around that. Really good. Not the quietest but really fruity and awesome! 

The best sounding cheap preamp you can buy is the Chinese copy of FM Acoustic 155 preamp I believe the patents on that design have expired so anybody can copy them and the Weiliang/Breeze Audio copies are very close to the real thing which cost around $20,000.Except they are more like $300.Very natural,open and 3D sounding but also very high resolution without sounding sterile.Exactly how good preamps should sound but most don't.

For $500 or $600 the MFA or Audible Illusion if you can find one is a nice choice. I had a Bruce Moore Companion long ago and for the money was very nine 6922 preamp. 

I think it would be real hard to find a MFA for that kind of money, they have gone up a bit in the last few years. You are probably looking at $1000+ now.  There is one on eb right now for $400 but it needs repair.   However preamps are not that complicated and probably $250-$300 in redoing and you'd have an excellent piece. .You can get a real nice Onkyo p308 in your price range and it’s a terrific pre amp