Has anyone upgraded power cord connectors?

Ernie: I beg to differ with you on the receptacles being "remarkably identical". There are pretty substantial "build quality" differences between, say, the Arrow Hart (Cooper), Hubbell and Pass and Seymour and what I would say are major differences in the blade configuration of these receptacles. This difference in blade configuration or contacts, along with possibly the materials the contacts are made of (although Bob says the P&S and Hubbell both use the same 688 Olin brass and I have no reason to disbelieve him) probably contribute most to the difference in sound quality between the various receptacles.

Ernie: Look closely at the contacts. Both the Arrow Hart and Pass and Seymour use (even in their 15 amp rated receptacles) a contact that can be, and is used in their 20 amp receptacles, allowing either a 15 OR 20 amp male plug to be inserted. Furthermore, the AH and P&S both use contacts that, if flipped around, would still work exactly the same way. The result is that the both the "live" and "neutral" contact accepting a 15 amp male plug (which probably 90% of users are going to have) are only going to make full contact along one side of that blade that is inserted. On the other side (again, when you are using a 15 amp male plug) of the contact, which is designed to accept a 20 amp plug, the surface area of the contact touching the blade of the male plug is going to be minimal.

The Hubbell contacts are totally different. If they are flipped by mistake during manufacture, they simply will not accept a 20 amp male plug. The "live" side of the contact in the receptacle is identical on both sides of the contact, resulting in a more complete and full connection (via surface area of both sides of the contact) gripping both sides of the male plug being inserted. The "neutral" contact in the 20 amp Hubbells is still going to suffer the connection problems of the Arrow Hart and P&S when a 15 amp male plug is used, but I am guessing the neutral side is less important, although I've wondered about this too, and think that for 15 amp male plugs, the 15 amp Hubbell receptacles may be even better, because, unlike the Pass and Seymour or Arrow Hart, the 15 amp Hubbells do not use contacts that will (if the face of the receptacle had the horizontal slot) accept a 20 amp plug. In the 15 amp Hubbells (ie. 5262), both the live and neutral side contacts grip the blade of the male plug fully along both sides of the plug. Thus, unless you're inserting a 20 amp male plug (how many audiophiles are really doing this?), the Hubbell 5262 may actually be the best choice, at least if the neutral side has anything to do with sound quality.

Apart from that, to me, the Hubbell's are overbuilt compared to the others and more solid. But I see this fairly substantial difference in the contacts as being pretty significant and probably the main reason why the Hubbell's perform so well.