Recommend a Used Solid State Preamp for About $5k

Please submit your recommendations for a used solid state preamp for approx $4-6K.

In your recommendations, please state the merits of the unit you suggest.

I am about to purchase a pair of Pass Labs X-600 monos and I'd like to try a solid state preamp with them.

I'm currently using an Audio Research Ref2 mk2. I'd like to do a side by side comparison with a solid state preamp in that price range to see which one I would prefer.

......the rest of my system:

Infinity Kappa 9 speakers
Audio Research Ref2 mk2 preamp
Audio Research CD2
Audio Research PH3-SE
Grado Platinum cartridge
Sansui TU-X1 tuner
Signal Cables throughout

thanks,

mitch