Swap out 12bz7 in favor of 12ax7?

I suppose what I should have done in the first place is to call Cary and see what they had to say about it, so I did. Marc, their V12 guru, says that the 12bz7 and 12ax7 both have a mu of about 100 and so would have similar gain characteristics. To reduce the gain he suggests a 5751 with a mu of 70, or a 6027A (12ay7) with a mu of 40, and he went on to suggest that he really liked the sound qualities of the latter. He also tells me that the power supply upgrades that he installed in my amp this morning sound great and it's putting out a solid 120 watts in UL, so I'm looking forward to getting it back sometime next week. Will experiment and update.

I found most 5751 tubes to be a little "dryer" or less tubey than 12ax7s. Let me see what I have laying around for you.
Did you ask him about 7189 instead of EL84?

The EL84s aren't really in the signal chain. He said the 12BZ7s would be a better place to start.

Sorry, I transposed that tube number. It should be a 6072A.

Just to follow up on this. I got my amp back today from Cary and did a quick A/B comparison between the 12bz7 and the 12ay7s and, indeed, the output volume with the 12ay7's is lower than the 12bz7's, requiring a little more volume on the pre-amp. Didn't have an opportunity to do any extended listening, but there was very little to differentiate the sound quality of between the tubes. I'll look forward to a lengthier session.