I made a rather large omission when I forgot to point out that although the 12AT7 input tubes may not wear out in the electrical sense as quickly as the driver and power tubes do, they are subject to deterioration in the area of microphonics, a problem which if it develops can severely compromise the overall sound of the amps. More than once has been the time my VTL's began to sound vaguely disappointing compared with memory or a solid state reference amp, but still it might take several days to dawn on me that the input tubes had started to exhibit audible resonances - D'oh! Replacement with a quiet pair immediately tightens everything back up and eliminates the creeping sensation that the presentation is beginning to sound confusingly frayed rather than solidly wholistic.
You can check these tubes simply by tapping gently on the glass envelope in various spots with something like a pencil. (Please feel free to cheerfully ignore those who admonish that you 'shouldn't' do this - it won't do any harm as long as you don't go crazy, and it is positively diagnostic, though it's true that a negative test result can't be taken to mean there still couldn't be problems). Any ringing or rattliing heard through the speakers means time to replace, and the louder and more lingering the audible results, the sooner the better. A good tube should display near silence when tapped. I recommend doing this test routinely every time you check the bias settings on the output tubes, since the problem can develop slowly and the deleterious effects are insidious but subtle at first. I would also do this test once a week for about 6 weeks after first installing a fresh pair, because you will want to replace new tubes under warranty if they show any incipient microphony during the break-in period. And although they're much less prone to developing an audible problem in this area, go ahead and do the same test on the driver tubes while you're at it, as they can occasionally become physically noisy too.
Along those same lines, I also recommend that anyone with these or similar amps make a small investment in some kind of resonance-damping rings for the input tubes, at least the inexpensive silicone types if not something more elaborate. They're cheap, they help, and many amp makers now install them as standard at the factory. Anything you can do to cut down on resonance in the input amplifier tubes will likely result in cleaner transient reproduction, more even frequency response with less tendency toward increasing brightness and forwardness under dynamic conditions, and more stable and coherent soundstaging and imaging with an overall greater sense of 'calm' and ease.
