You see the problem here is one of credibility.
Whilst I'm struggling to understand how a USB cable could possibly affect sound quality, the rest of what you've suggested is just conjecture.
- You've suggested that your experience with headphones allows you to guestimate sound increases with normal stereo systems. My experience is that it's simply not the case. A number of years ago, I looked at simplifying my system and going with only cans. I borrowed a headphone amp to use with my HD600s and also a Stax system. After 2 days of comparing, I returned both as for me, neither got close to my system, which back then was reasonable, but nothing close to what I now use
- You've stated that a £10k system is not really better, with seemingly zero experience. If you're genuinely interested, I don't know why you've not bothered. It's really not hard, just visit a decent show, some good dealers or if you have the chance, visit some people who have higher priced systems. Don't get me wrong, I've come across some really bad sounding systems regardless of price. However, I've also heard some that sound utterly sublime and make my own sound a little broken in comparison. At least you'll then have a reference point to understand what it's important to you (as we're not all the same)
Right now, you're just throwing stones over the wall, with pretty much bugger all credibility. It's like a Corsa driver trying to tell someone with a TVR why it's pointless.