Tom,
I might have given the impression that I know more about this than I actually do. But from what I've read, there is generally less 'grain clumping' with a lower pH. I assume 'grain clumping' to be the grainy quality we see in the print. D76 has a pH around 8.6, XTOL about 8.2; XTOL does in fact give slightly finer grain than D76. Microdol-X and Perceptol are about 7.6, undiluted, so you would expect either to give finer grain than D76 or XTOL, and this does seem to be the case. (Microdol-X and Perceptol are apparently similar, although not identical, formulas).
What happens to the developer pH when diluted? I'm not sure, but my guess is that there should not be a great change with the usual dilutions.
'Which version of D76'? I think the standard formula gives a pH of 8.6, after it has settled down for a day or two. However, the Kodak version is different and the pH is slightly lower.
That's about all I can say. I hope someone will jump in at this point.