Ah, that's the 'death spot'!... You should send your amps to me for safe disposal as soon as possible.
Or... you could just keep on enjoying them and not worry too much about what looks like a painted marking on the glass which has discoloured due to the heat. Seriously, that's all it looks like form here. If the amps have the facility to check/adjust the bias it may be worth checking that it's in spec and doing a few regular re-checks just to make sure things aren't going awry.
Shian7
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Kudakutemo
kudakutemo
ari mizu-no tsuki
Though it be be broken -
broken again - still it's there:
the moon on the water.
- Choshu.