Dear Sbrown, Was just passing by and saw this old thread. With regard to your chosen tube complement, just be certain that the "second gain stage" does not lie within the RIAA network, because the electrical characteristics of the 5751 are not identical to those of the 12AX7. Thus using a 5751 would alter your RIAA correction. (But perhaps this has had a sonically pleasing result, by chance.)
Aesthetix can tell you whether this is an "acceptable" tube substitution.
Aesthetix can tell you whether this is an "acceptable" tube substitution.