If both mono amps, (of any make) are biased correctly and matched as a pair, it should make no difference in sound.
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Are the amps the same make and model? If so, then it's not indicative of a bias issue. The sound stage will seem to migrate to the right channel when the amps are not the same regardless which is biased higher ime. Could be you had a dirty or poor ic contact and just reinstalling cleared it. Re-swapping will reveal that.
I leave my systems powered on except during thunderstorm season. I have read that during break-in of any component, powering it off for 30 minutes will re-align the molecular structure, and I think I experience this phenomenon where the sound is improved after the system is powered up again. If this is actually true, then it might also enhance systems that are already broken in and left powered on for long periods of time.