How much of a compromise would I be making if I used unbalanced interconnects seeing as the Pass amp is a fully balanced design?Great amp. In my view the preamp is one of the most important and consequential pieces of equipment in an audio system, so I wouldn’t get a lesser pre just to maintain a balanced connection. It’s expensive to implement a truly balanced preamp, so with a $2k budget you’re asking a lot IMHO.
That said, for tubes a Sachs pre can be had fully optioned up for about $2500 and a Linear Tube Audio MZ2 for $2140 fully loaded, and both offer a return policy so you can try either in your system, which is really nice. Both are single ended only, but hard to see being disappointed with either sonically. Don’t know your sonic preferences, but the Sachs is probably the slightly warmer of the two while the LTA likely a little cooler by comparison with both skewing more to the neutral than not so depends more on which way you’d like to lean (of course choice of tubes could affect this significantly).
If you’re stuck on a balanced design, a used Atmasphere MP3 would be worth a look and is fully balanced although not all come with a remote. A used Backert Rhumba 1.3 can also be had used for around $2k and has balanced connections, but I don’t know if it’s balanced from input to output. Anyway, hope this at least gives you some options to chew on, and best of luck in your search.