Sound variability in mid-day = differential noise?

That's my thinking as:
1) common noise (from RFI / EMI) would be removed via my AR RTP power conditioner (which works very well btw)
2) AC voltage irregularities (overvoltages/sags) are a red herring as all components audio sections operate on regulated DC--which is what the components' power supply does
3) harmonic disturbances (corrected via inductors) are inoccuous, as the treatment (inductor based conditioners) make things worse

so, if that's the case, what products remove differential noise (defined as noise levels different b/w line & neutral, generated via switching power supplies also on line---air conditioners, band saws, fluorescent lights, appliances, etc). AFAIK balanced power still wont fix this problem.

Barring power regeneration, differential noise is dealt with via filtering.
Not all sections of all audio components are regulated at all, never mind 100% immune to voltage variations. Lots of output stages for examples have unregulated rails.

You might want to check your voltage with a cheap multimeter before spending big bucks to eliminate noise that may not be a problem.