Interesting input so far guys, thanks. I've been doing some reading myself but sometimes it helps to actually have the conversations.
Just a few things I wanna note.
I don't think it's necessarily straining. 550w would usually be sufficient headroom for this build. The voltage readouts for all rails are near-perfect, too. If there's any strain at all, I'm not seeing signs of it, even doing tests under heavy load. From the moment I boot up it's there and it stays that way. The sound doesn't even change if I try to max both CPU and GPU. The only reason I assume 5v rail is suspect is because it is carrying over to the USB bus and it doesn't seem to care if that interface and my mouse/KB are the only things on it. Maybe it's just carrying through everywhere. But it was just a passing observation to begin with.
Experiences recently are telling me that the USB bus is just generally finicky when it comes to input power. A badly shielded outlet gave me some real trouble there recently. Somebody mentioned that if it's carrying through to the speakers it's something else, but my experience says that all it takes is to have bad power going in and USB audio can pick up all sorts of nastyness, especially USB-powered stuff, such as my interface. I'd believe that nothing is wrong with anything else in this rig. Especially since I can clearly hear the whine both coming straight from the PSU and being directly mimicked through the speakers, ONLY when they're plugged into a bus-powered device.
I also have a USB/optical DAC that is self powered feeding these same speakers. Doesn't pick up the whine with either input... ...and it's only got unbalanced outs. So that right there narrows it down. As soon as I plug in that powered interface (or if past experience still stands, ANY usb powered interface - I've tried several - all with the same susceptibility to power noise,) I'm going to hear the noise passing right through balanced outs to the speakers. Last time, dealing with the bad outlet, I tried swapping every component in this thing with parts from my B-rig, after verifying that the B-rig was in fact noise-free. Different case, no case, different GPU, different PSU, no unneeded stuff, different mobo, etc. I mean, I tried everything. Ferrites, isolators, ground lifts, all manner of cables, hubs that use their own wall power... didn't matter because that outlet was just bad and bad power going into the mobo was enough. Ultimately I did have a bad mobo too, but I swapped that and that problem fully went away. The current components in this rig did not have this issue. It was when I put in the Seasonic that I started having problems. Doesn't mean there aren't other susceptibilities in say, the mobo - perhaps a better mobo wouldn't pick this up, but at this point the source is clear.
I can see the headroom argument. A higher price, higher wattage unit may very well be what it comes down to, or at least it takes another potential factor out of the equation. But my concern there is that even people with units like that sometimes experience whine. Some are even sort of known for it. Seems like more a matter of how the coil is placed and wrapped. A whiny coil isn't necessarily bad, and for most people a little whine isn't as much of an issue as consistency and reliability, which aren't necessarily affected. I'm willing to believe that almost all PSU's exhibit whine, and it's just a matter of whether you can hear it or not. Obviously construction matters there, though.
But still, I think I'd be more willing to try a higher wattage, higher-quality unit over a cheaper one at this point. As mentioned, sound is a major concern and I am willing to pay more than I otherwise would to take care of it, even if I'm technically "over-spec." I know all too well as someone who's worked on amplifiers that good sound starts with good power. I'm just not sure that simply picking a good PSU with known good design/components will be enough. It's one of those weird, specific things where it's hard to know what to look for to know you're getting what you want. Hell, half of the time it's just luck, though looking at some of the comments here I might be ruling out Seasonic. Maybe Corsair, too. Depends on what I can turn up about the ones I'm looking at. I dunno. Doesn't cost me anything but time to give a replacement Seasonic a shot, I suppose.
I've learned their RMA system is amazingly good if you stay with them, too. Go for the amazon credit and you get it basically the moment your package scans out. But then that deal pops up on newegg and I find myself torn.
