If I was you with your system I’d definitely try a tube preamp if you’re open to that. You’re most likely to hear bigger differences (and benefits) versus your Krell, and the tubes tend to last a long time in preamps and typically not very expensive so are relatively low cost/maintenance components. The Backert Rhumba 1.4 would be well worth trying, and they offer a 14-day trial period.
https://backertlabs.com/rhumba-1-3-tube-preamplifier/
If you don’t need a fully balanced pre (but available with optional balanced inputs) the Linear Tube Audio MicroZOTL would be another great option and also offers a trial period.
https://www.lineartubeaudio.com/products/microzotl-preamplifier
I’ll also mention the Aric Audio Motherload II that is not balanced but not sure if they offer a trial period.
https://aricaudio.com/products.php?product=MLII
All of these would likely provide a very ear-opening experience — I’d be shocked if you didn’t hear very meaningful differences — if you’re up for it. Just my $0.02 FWIW.
I also agree room treatments are worth exploring, and your Oppo 105 could also certainly be improved upon just by adding a better DAC. It never ends.