would finding a preamp with direct coupled outputs solve this issue ? The MP-3 MK3 is designed this way and I see some other preamps heading this way..The direct coupled output of the MP-3 would have no trouble driving both your amp and a powered sub, if it could be modified to provide RCA output connectors in addition to its XLR connectors. I’d imagine that in general that would also be true of other tube-based models having direct coupled outputs, but I’m not aware of any such designs other than Ralph’s. And I think the design of his direct coupled output circuit may be patented.
But there are certainly a number of tube-based designs having capacitively coupled outputs or transformer coupled outputs that would also be suitable in that respect. Offhand, though, I can’t think of any that I know would also meet your other requirements and preferences (balanced and unbalanced inputs and outputs; balanced internal signal path; tube-based; ~$4K or less).
Regards,
-- Al