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.

A good tube CD player may just be the ticket. You can roll tubes to tailor the sound to your liking. Romulus would be a great choice I think.

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.

I have been seriously considering a tube player. What are your views on the Romulus & CD7?

The Romulus is a real killer.

Add, Parasound CD-1, Audio Note 4.1, ARC CD5 or CD7.
These players will get you started. I love cd/sacd players and every year the tech gets better, so, there are many, many excellent to outstanding models in the market.

Go listen for yourself, do not cheat yourself.
Happy Listening!

If it's just the upper mids that are bothersome and you're otherwise happy with what you're hearing, another way to go that may work and potentially be much cheaper would be to try some other cables and interconnects. Nordost are great to be sure, but even in the context of otherwise very good systems they sometimes don't convey enough tonal density or weight or can be a little too revealing in certain areas. If it were me, before dropping big coin on a new source I'd try to audition some Acoustic Zen Silver Ref. II interconnects and Hologram or Satori cables to see if they might yield improvements in the upper mids without sacrificing in other areas. Obviously there are lots of other brands you could try, but I only mention AZ because I'm most familiar with them and think their sonic signature may work particularly well in your situation. If it doesn't work at least you can go forward with more confidence you're doing the right thing.

Final thought -- if you have a Bel Canto dealer nearby maybe try the CD1 feeding a DAC3.5VB and see if that works. I found the DAC3.0VB to be much better than the CD2 in my system. Hope this helps and best of luck.