Upgrade on Bel Canto CD-1

"I listen to mainly progressive technical metal & post rock."

I don't know if upgrading to one of the CD players you list is a smart move. They're at least as detailed as your current CD player. I think there's a bigger chance that you'll make the problem worse, not better. Given that your problem is with just some of your CD's, I have a different solution for you that will save a lot of money. You can leave your system just as it is and get a digital eq, like a Behringer 2496. Connect it to one of the digital outputs on your CD player, and a free input on your preamp. Use the eq to make your bad recordings listenable. Having it on a different input than your CD player lets you eliminate it completely from your system when you're not using it.

Trying to fix problem recordings by getting new components is almost impossible. Not every CD giving you trouble will need the exact same thing to fix it. Its like buying an eq with 1 setting.

"09-29-14: Infection
The specific sections of the discs which are problematic are not to do with an inferior recording. It was not apparent with my previous player."

Actually, its most likely the recordings.

"I don't know if upgrading to one of the CD players you list is a smart move. They're at least as detailed as your current CD player. I think there's a bigger chance that you'll make the problem worse, not better."

That's exactly what I was talking about in the above quote. You're hearing more flaws with your new CD player.

"I've just tried my Artisan Ultimate Silver Dream XLR & there is an improvement with the upper mids, although the problem still exists in other sections & it's lost a little space/air around the instruments."

That was another point I was trying to make with my comment on buying components to fix problems is like getting an eq with 1 setting. You improved the upper mids but at the expense of air and space around the instruments. Its just as easy to lose, as it is to gain. Always the tradeoffs.