That's a very good question. And the answer isn't so straightforward.
Generally, over-voltage on a tube causes sudden failure early on in a tube's lifetime. Typically, the warning signs will be crackling noises and even sparks appearing inside the tube. The corollary of this is that high voltage testing is a great way to weed out tubes that have manufacturing defects in them.
Deterioration of a tube in the longer-term is more usually accelerated by exceeding dissipation limits, rather than high voltages, per se. However, in ordernot to exceed dissipation limits, as voltage is increased, current has to be correspondingly decreased.
In a typical Hiwatt, the preamp and PI tubes are set to idle at around 150 to 200Vish, despite the fact that they're running off rail voltages in the order of 350 to 430V. This appears to be sufficient for the preamp/PI tubes to have a long and useful life. The one thing to be careful of is that the cathode voltage of cathode follower and PI tubes doesn't exceed their cathode-heater breakdown voltage threshold, which is something that tends to vary between different brands of 12A_7.
Next we need to consider the power tubes. With these we also have to be careful to limit screen voltage and screen current, especially on large signal swings, so that the screen grids don't exceed their dissipation limits and head for melt-down. The screen grids are a lot more fragile than the anodes for fairly obvious reasons. Not only can they be melted far more easily, like any grid arc-over inside tubes can destroy them.
In the case of Hiwatt amps, it's important to note that they have a track record of immense reliability, and provided reasonably good quality tubes are used, they last a good number of years . In fact reliability is one of the main reasons many bands would tour with Hiwatt amps, rather than Marshalls, which went through a period of suffering from reliability issues. These problems were caused by overhigh voltages, combined with inadequate protection of screen grids, plus transformers which weren't very robust due to being made to a budget price. So blown transformers and fried power tubes happened a little too often with the Marshalls, while the Hiwatts just kept on going.
In the case of the Triwatt, we have to run at somewhat lower voltages than typical vintage Hiwatts, due to the fact that we're using 6V6s. However, these voltages are still high enough to get the tough clean tones and the buzzsaw like overdrive of the big Hiwatts. The Triwatt also has extra protection on the screen grids compared to the big Hiwatts, so that the 6V6 screen grids don't get melted when the amp gets cranking at voltages which are relatively high for 6V6s. This little extra protection doesn't hurt the KT66s either, although they are running comfortably below their limits.
HTH
bgroup wrote:
Might have been the best bass sound EVER!!!
Can't wait to try bass through the Triwatt!!!
Well, as you know, it will sound pretty darn similar, but not quite as loud.
But wouldn't your DR103 to the job better in terms of having a bit of volume to hold up the low end?
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Not a good thing to do near any high-voltage tube amp! 
