Speaking of dropping voltage, the 6X4 seems to drop more voltage than the 6Z4 did since the B+ now measures around 195VDC instead of around 220VDC. Since the HV was too high due to the 110V transformer anyways, this will probably be for the best. I really need to find out what B+ this amp is supposed to have and go from there.
LOL! I was actually thinking last night how easy it would be to do this mod and forget this fact. That would be a bummer for sure since your amp would down until you got a 6X4.
Just as a joke (since I wouldn't waist the time to ever go back to the 6Z4) you could install a DPDT switch on the back panel allowing switching back and forth between the 6Z4 and 6X4.