Power out of my conditioners is a rock solid 120v, although I have not checked this under high volume conditions.
Power conditioners? Them must be some big honken power conditioners.
What is the KVA rating of each power conditioner?
What is the quality of the receptacle the amp/s are plugged into on the output of the conditioners? There is where, at the receptacle, you need to check for VD.
Have you tried plugging the amps directly into the wall outlets?
With #10awg wire 4ft long from the main electrical panel, the power available for short quick draws of current could be well over a 100 amps. More than enough current available for the amps. The 20 amp breakers protecting the #10awg wire will pass quick short draws of current all day long with out tripping. (I assume the electrical service is at least 100 amps or 200 amps. Check the main breaker handle rating.)
The SPL meter sounds like a good idea.
Jim