I sent an email to Keith Herron, his prompt reply regarding the VTPH-2A and Ayre Ax-7:
" We have VTPH-2A customers that are using preamplifiers with 20k Ohms RCA single ended input impedances that are pleased with the sound.
I don't recommend going any lower than that. The 2 X 12AX7, 3 X 12AT7 version of the VTPH-2A has an output impedance of 400 Ohms so it will drive the Ayre preamplifier much better."
" We have VTPH-2A customers that are using preamplifiers with 20k Ohms RCA single ended input impedances that are pleased with the sound.
I don't recommend going any lower than that. The 2 X 12AX7, 3 X 12AT7 version of the VTPH-2A has an output impedance of 400 Ohms so it will drive the Ayre preamplifier much better."