I agree with cpk, give Andy at VTS a call. Also, you can check Music Reference/RAM Labs who also sell low noise and well matched tubes.
FWIW I just bought a Music Reference RM-10 MkII and it uses 12AX7 tubes. I can swap in 5751's, but to work optimally a couple minor changes to the circuit have to be made. So it's not always plug and play.
FWIW I just bought a Music Reference RM-10 MkII and it uses 12AX7 tubes. I can swap in 5751's, but to work optimally a couple minor changes to the circuit have to be made. So it's not always plug and play.