Looking At CD Players

I can comment on the Yamaha CD-S 2100 since you mentioned it specifically and not just because I own one...

It's a superb design and in addition to excellent sound quality offers a lot of flexibility. SACD, CD, USB DAC. The build quality is excellent. Its drive mechanism is something you'd expect to see in an Esoteric unit. Not a single piece of plastic in sight.

The only con i can come up with is toggling through the input selections is slower than expected. 

There are only a couple of pro reviews out there on the S2100, one from a new zealand magazine. Its pretty accurate and worth a search.

I forgot about this CD-S2100 review which offers some comparisons with the Oppo 105 you currently own:https://hometheaterreview.com/yamaha-cd-s2100-cdsacd-player-reviewed/
Here's the Aussie review  (sorry kiwis) I mentioned earlier. It's in PDF format:
http://i.nextmedia.com.au/Assets/yamaha_cd-s2100_cd_player_review_test.pdf
I recently watched a Darko review on the Hegel unit. While some of the YouTube reviewers lack integrity, Darko is a pretty straight shooter in my opinion. The Hegel looks like it could also be a nice step up from the Oppo.

Since Oppo is no longer produced, it seems like a poor decision to spend a lot of money modding it. Like modded cars, you'll never see that money back at resale time as buyers won't have any idea if your mods are going to be to their liking. As suggested by another poster, you're probably better off using the Oppo as a transport and running it through a DAC of your choice.

Let us know how you make out.