I live in a neighborhood of homes built between 1900-20. Elevated power lines on the same poles as cable TV and telephone lines. The power coming into my house varies between 118 and 126 volts with a thd between 4 & 5.7%. I have a dedicated 20 amp circuit. I noticed a huge improvement (lowering of the noise floor) when I bought a PS Audio Power Plant Premier. Output power is a contastant 120 volts and the tdh is reduced to between .2 and .6%. I plugged all source gear into the PPP. I also plugged my Music Fidelity A-5cr amp into the PPP.
When I replaced the MF amp with a ModWright KWA 150, the draw was too much for the PPP so I plugged it directly into the wall. The noise floor was elevated compared to when I ran everything thorough the PPP. So I replaced a PS Audio Powerport recepticle with a PS Audio Soloist Premier in wall conditioner and bought a PI Audio Uber Buss. The Uber is plugged into the Soloist and both the KWA 150 and the PPP are plugged into separate circuits in the Uber. Source gear remains plugged into the PPP. All power cords are after market. The result is a dead quiet (black) background/noise floor allowing more detail, dynamics, etc to flow from the speakers. The improvement was more than subtle and worth every penny.