Parallel?
What amplifiers?
Power conditioners are not actually for correcting obvious problems. They aren't for removing RF that you can hear in the speaker. The majority of obvious problems need to be fixed in a different way, unassociated with the AC power.
Power conditioners, what they actually do is reduce the stuff on the AC line that's affecting what you hear. You hear the removal of the affect, not a removal of a "sound" or "artifact". Simply... you should just notice you stereo doing everything better, and a reduction of fatique.
Fisher makes a little 'acoustic guitar' amplifier. If you play it into the wall it sounds fine with acoustic guitars. If you plug it into one of my musician conditioners then it sounds good electric or acoustic; but even more noticeable is the lack of a "splat" sound at the peak of every note replaced by a fully rounded note. Prior to that experiment no one knew there was a "splat" at the high end music store, but after it was gone you couldn't unhear it. The point being there wasn't an unwanted sound, or artifact (all of those were affecting amplification). It was the note the whole time, but the quality of it changed drastically even though the "sound" of the tuning, knobs, all that, was the same.
*explaining this to a musician is almost impossible, they're mostly insecure people scared that a power conditioner will change the type of sound their guitar makes... Because quality to them stands for the type of sound that's being made, not how well it's made.