Lewm,
Thanks for the detailed information on the various tube types. Yes, the 5751 is a lower gain tube, but in almost all applications it does work in place of a 12AX7 or 12AT7. But, I think it is also worth mentioning that tube swapping MAY be problematic with phonostages if RIAA equalization is not done passively.
By the way, I did not say that one-box preamps are inevitably inferior. I said that most of the ones I heard were not that impressive and noted that the Audionote M-8 is one exception. I think that the M-8 costs over $30k. I have, though I don't current use it, a Mark Levinson No. 32 with a built-in phonostage. Contrary to the opinion of reviewers, I think that that phonostage (built on two cards that slot into the back of the unit) is not very good (the No. 32 is a pretty expensive preamp). I've heard Shindo preamps that sounded very good, though I don't specifically recall whether the built-in phono was outstanding too.
You are correct that there is a big advantage to building in the phonostage -- a long run of interconnect cable and inevitably crappy RCA jack interface is eliminated. But, many of the better phonostages cannot be built into another component because they take up WAY too much space. For instance, my current phonostage has a separate power supply that is bigger than most power amplifiers (it uses two 300B tubes as rectifiers).
