If I remember correctly, if it's one tube per channel, essentially none, except for differing tube life times. If it's a parallel single ended amp with more than one tube per channel it depends on how each channel is set up.
If a cathode bias scheme is used and each tube has it's own filament transformer, than same as the single tube per channel set up.
If multiple tubes are running on one filament transformer, then mismatched tubes will generally wear at unequal rates.
This is my experience and (limited) conceptual understanding.
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