Pass labs XO.2 or Janus tube pre-amp.

Gary,
Matching the Pass with a tubed preamp is a good idea. Like you said, you don't want to give-up the bass, and you probably don't want to give up that silent background. Some tube preamps will really enhance the midrange and 3-dimensional aspect at the detriment of the first two.

It's really best to arrange for a demo, or buy a piece of equipment with good value and resale. Matching amps and preamps from different companies can be rewarding, but also a little risky.

The BAT fits that role for me. It gives-up a tiny bit on the bass c/w running direct, but it is far more satisfying overall. I'm not sure how good their phono section would be.

Another option to consider would be a move to the XA.5 series.

You will need to find out what the output impedance of the Janus pre is spec'ed at. Some tube pres have a fairly high one and Pass amps have a relatively low input impedance. You would want the input impedance of an amp to be 10 times greater (or more, general rule) than the preamps spec.

Right now I am listening to a Conrad Johnson tube pre going into a Pass XA30.5 with excellent results. The CJ (a CT-5) has an output of 800 ohms and the Pass's rca input is close to 20k (I have to use the rca's) so I'm at a 25 to 1 ratio.

Chances are good that you will be fine unless the Janus output is abnormally high.