Pass Labs XP-25 Outputs

Keep in mind, your TVC linestage is a passive device. It adds no gain to the signal.  Most active linestages can add at least 10db or more of gain. So you are fully dependent upon phono stage gain.  Like benjie says, you need to up the gain setting on your Pass XP25.

True balanced circuit vs single-ended circuit will if properly designed have 6db more gain than its equivalent SE circuit, all other things being equal.  Which is to say that if you ground one phase of a balanced circuit, you get an SE circuit with 6db less gain.

I would consult Pass Labs before doing that.  Is there a switch somewhere that selects single-ended vs balanced output?  If so, maybe.  If not, then don't.  But do consult them.

I have a question.  You wrote, "I am currently trying out a Pass Labs XP-25 phono stage and enjoying what I am hearing, particularly with an MC Step Up Transformer feeding into it vice using the MC settings available."  I suspect there is a typo in here somewhere.  Tell me you are not running an external SUT into the MC inputs, but if you are, it's surprising if you like the result.