Call Keith and ask him.
Keith Herron has moved to Electro Harmonix tubes at least in his pre-amps. I have the current version (v.2) of the VTSP-2, with 6 EH 6922 tubes. Mr Herron has been more than willing to discuss tubes and any other features of his line with me - he is a true gentleman. I just chatted with him 2 days ago about new NOS tubes (Amperex USN-CEP 7308's) for my pre-amp and there was no mention of warranty voiding.
By the way, the new phono preamp, VTPH-2 is now available, listed on the website. One day, when I get back into vinyl, I'll be checking it out.