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Push-Pull Tube Amps for Dummies: Questions 1 & 2

Posted by immatthewj on July 25, 2020 at 22:45:08:
I've been listening to tubed stuff for quite a few years now, but I really don't understand much about them other than changing tubes when they go bad, rolling tubes to make myself even more neurotic than I already am, & adjusting bias.
On the subject of bias, what effect does bias have on what one hears from the speaker? I have noted on a previous amp that I used to listen to, that as the output tubes started sounding old & tired, the bias would start slipping downward & I would correct the problem by screwing the bias pot in until I broke down & replaced the tubes. But assuming that the tubes were fresh & serviceable, what effect would
1) less bias current than called for
or
2) more bias current than called for
have on the sonic effect heard from the speakers?
Question 2 is: what is the relationship between a push-pull amp's power & the number of output tubes it runs with? This question does seem like a no-brainer (I did title this as a "for dummies" question) but for example, I once owned a pair of mono-blocks that ran with a total of 8 6550s (4 per monoblock) that was advertised at 110 wpc in class AB. I now own a stereo amp that runs with 12 EL34s & is advertised as producing 100 wpc in AB "pentode". There was this amp called "The Mesa Barron" (with a total of 12 5881s) that I once took home to try out that said it made 150 wpc in AB "pentode". Is this all due to the difference between the output tubes, or is there something else that is different going on inside the amps?
Thanks/Matt