Yesterday I had two important upgrades to my electrical system for audio.
#1 my house is now actually grounded (the old ground rod had gotten disconnected decades ago) with a new ground rod connected with 4 gauge wire (twice the diameter of the previous 8 gauge wire which connected to nothing).
#2 I replaced the Oyaide purple/gold outlet with a NOS Pass & Seymore 5262A (the legendary outlet recommended by the late Bob Crump). The 5262A has much more consistent grip, holds plugs straight into the wall, whereas the Oyaide would always let them sag a bit.
Since these were both done during the same visit by electrician, I can't separate the effects (though I think the outlet upgrade was less important, I had previously found a trick to maximize performance of the Oyade). But combined, the effect was that a piece I had been listening to previously, the Bach 4 Organ Concertos by E Power Biggs on SACD, went from being rather unlistenable to satisfactory (though it's still pretty noisy).
In addition, I added a new whole-house surge protector. This is a new Intermatic device which shunts across the power lines (not in series) through a 50A breaker. I did not expect this to have any positive effects on audio sound quality, but hopefully it will help preserve all my electronic equipment, including amplifiers which I often plug straight-into-the-wall with no surge-protecting outlet strip. I would certainly not plug my Krell into such a strip, it might blow up under the 20+ amp instantaneous draw of the Krell. (Just because it's on a 20A line, doesn't mean instantenous draw is limited to 20A.)
At least the whole house surge protector did not have any negative affects on audio I had detected so far.