As you are using a balanced Pass amp, you may want to try a balanced Pre to lower the noce floor by taking advantage of common mode rejection. If you were open to solid state devices, I am extremely fond of the Rowland Capri: $2700 new, and approx $2K used. Yes, the noise floor is incredibly low, and volume increase is in 0.5dB increments. Device has a unity gain input if you want to bypass its volume control all together. See specs at:
http://jeffrowland.com/CapriPreamp.htm
I prefer it over my own ARC ref 3. Guido