Mribob,
The allure of tube rolling is a force, the temptation great. One can almost change, with a new tube, the overall sound presentation of their preamp. However, if the manual states, 'do NOT'--before considering a change, I'd want to personally talk to Kevin Hayes and find out what, if any effect it would have on the warrany...and moreover why he's opposed to doing so. Knowing Kevin, there are likely, very valid reasons for him opposing the change.
It won't hurt to ask, and at worst you're 'stuck' with the great sound you already know.
Mribob, don't you just love the voicing of VAC?
Good listening,
Larry