Manual vs. Auto Bias

More over I'd say that if I ever go back to tubes, I'll make sure I'll buy an auto-bias amp or if price is convincably low for fixed-bias unit, I'll DIY implement it(most-likely with SS parts for less space and money).

Autobias has a main advantage over fixed bias that tube life is much longer than in fixed bias. During powering up an auto-bias self-adjusts the quiescent current thus giving a tube a 'soft' start.
Sonically It's also more beneficial since stabilization of quiescent current decreases distortion.

The same amp that has manual bias can be upgraded to have an autobias by quite a few examples from simple shared cathode resistor to a complexed current mirror (using transistors, zenner, shottke diodes). After such upgrade the sound does not degrade at all in the same amp. The cost of labor increases and probably steals the manufacturer's profit, hence there's a reason to say that it can degrade the sound.

Do you know that 2 matched new tubes will age differently and even if you check bias of your amp often, you might still miss the moment when tube's going out of bias beyond its operational tolerance even before reaching its half-life? I'm very savyy with DMM of any kind, but learned to appreciate tube amps with autobias implemented.