I honestly cannot give you a scientific or even logical reason for what I heard, but I can tell you that different USB ports (connected to difference hub locations on the motherboard) had slightly different sound -- just like using in on my windows 8.1 netbook produced a different sound than my hand built (using best parts at the time) Win 7 PC. But I honestly think that is a processor issue rather than a Bifrost issue.
Not knockin the Bifrost, just it's user friendliness to set it and forget it and get the best sound out of it.
Correct me if I'm wrong, but if you're using USB, that's 100% digital. Meaning port won't matter. Like if 100 people download an mp3 from a website, and we all run a checksum on it, we'd get the same hash because we have the EXACT digital copy of the song. Not one single bit would differ between our copies of the file. Same way digital music transfer works.
Now if you had five of the same soundcards all in different PCI slots, and use analog output to your amp, then yes, that could sound different. One card could be closer to your video card. One card could be on a different power rail that's cleaner. Analog signals are susceptible to noise.
It's like how digital TV works or it doesn't. If it's working, you're getting the exact picture everyone else is. (granted your TV will render the colors, brightness, hue, etc differently, but the source image is digitally identical to the version everyone else is receiving because it's digital)
Reminds me of the fools who spend $1500 on a 6' HDMI cable. Unless you're running it across your house in the wall along-side your mains wiring, there's zero benefit to getting a crazy shielded cable. It's a digital signal. It gets there in-tact, or it doesn't. If it doesn't, your run is too long or too noisy, in which case you'd actually benefit from a better cable. But in most cases, it's moot. Snake oil is a common theme in theseparts! Makes me want to buy two top end TV's, put them side-by-side, use a $3 monoprice 3 foot HDMI cable on one TV, and a $1500 snake oil HDMI cable on the other, and see if anyone can ACTUALLY tell a difference. Does digital ever get noise? Yes. In the form of lost packets during transmission. What would it look like for a TV? Have you seen how TV gets all blocky when a cloud goes over your house and you have satellite TV? That. That's what it looks like. Same with your music. It works or it doesn't. I'm 99.9999% sure your sound difference based on usb port is in your head.