Differences in electric outlets

Electroid and Rja have nailed this one. The Oyaide R1 is the audiophile outlet to have. (I tried Hubbell, PS Audio, and Furutech before).

Power outlets do make a noticeable difference in sound.
So do the plug ends on your power cords.
Think of them as interconnect ends for your power connections.
Clean power to your equipment is just as important as your interconnects and speaker cable.
Ironically, one of the best sounding plug ends is the cheap Leviton three prong plug ends from your hardware store.
These are the ones for around $10, and have a white nylon insert on the three prong end.
I replaced the end of an MIT Z-cord with the Leviton, and it sounded significantly better.
One of my local high end audio dealers was making his own power cords using Leviton end plugs, and I still run his cords today on my power line conditioner and preamp.
The high end manufacturers know about it, too.
Did you know that Synergistic Research power cords use these cheap Leviton plug ends?
They heat shrink the entire plug end so you can't tell what it is!
I looked closely at two of my Synergistic Research power cord ends, and sure enough, the tell tale signature Leviton plastic ends and screws were there.

At my old job we ran playback equipment on a dedicated 20 amp feed using an isolated ground and power that is filtered through a bank of batteries to remove ripple prior to being rectified back into AC. Clean power is critical to good sound. No arguement. We did an interesting experiment using pink noise and analysis software to see if there was any change in measurement when changing just the electrical ends on the amp cord and receptacle. Proving me wrong, a good, clean connection does make an measurable, audible difference. It's so small any ambient change such as room temperature or humidity would negate it, but it is there. If you're worried about power spend the money on a good conditioner and buy whatever ends make the best match to your gear. FWIW, if you want to swap both male and female, Hubbel hospital grade male to female did make a measurable difference. I personally run an isolated ground dedicated cicuit at home using Leviton ends everywhere. Best $200 ever spent on my system.