[Bret L]

One concept most people do not keep in mind is that just because a
unit has XLR or any other multi pin connector that does not mean the
signal is balanced. Most of the prosumer recording gear sold today
uses XLRs in unbalanced or quasi balanced configurations, and a good
deal of high end equipment does as well.

For those too lazy to terminate their own cables this can cause
substantial problems since some of this stuff is "Pin 2 Hot" and some
"Pin 3 Hot". There is also the issue of whether the unit is inverting
or noninverting, which if you consider absolute polarity important can
really make your day.

You also need to consider whether the unit is designed for -10 or +4
dbm full output operation.

There is, however, no doubt that the XLR is miles above the stinky
RCA as a connector.

Another design issue is this: with tube circuits, true full balanced
operation means open loop because in balanced circuits with NFB,
failure or gross mismatch of one tube section means that you have an
oscillator on your hands. Plus, you need a lot of tubes at any rate.
Most classic pro tube gear was single ended internally and interfaced
to the outside with transformers.

I suspect the OP will wind up settling for less than his noble
original concept.