My Oppo is a bit too bright

It will make a good transport. Depending on your budget, the Schiit Dacs have a good rep as does the Denafrips Ares II at around $700.
Your coax cable makes a difference. The mellowest coax digital cable I've found is the Audio Advisor Penangea for $95. Belden's standard coax at $20 is also a good cable. I've tried the expensive cables and you can expect more detail but attendant brightness and more of what you don't like. 
Also Have you done the latest Firmware update on your Oppo. They still have a legacy web site and can upgrade your unit They sent me a DVD to upgrade my Oppo 93 software. 

I have several CD players but I use my Oppo 105D more than the others mainly for the features available via the remote, like volume. Also, it has a solid, competent sound. I bought a Benchmark DAC1 many years ago, but never got around to using it until I bought a pair of Audeze LCD-X, closed back phones. When I hooked up the DAC to my Oppo, I was shocked at how much it improved the sound. I shouldn’t be surprised as the Oppo cost around $1400, and handles nearly every type of disc, while the Benchmark cost $999 and simply converts the digital signal, but does it very well. Forget that crude comparison and just believe me that a good DAC (like Benchmark) will tame that bright sound of your Oppo.

What was the cost of your digital versus your vinyl setup?  Not really a fair comparison now, is it?  Sheesh.  If you’re at all serious about getting better digital sound, invest in a better separate streamer and DAC.  If it was me, and since you do more streaming than disk spinning these days anyway, I’d ditch the transport altogether and get an Innuos Zen Mk.3 streamer/server and just load all my CDs into it.  Then I’d get something like an MHDT Orchid or Audio Mirror Tubadour SE DAC and another set of AZ interconnects to link it to your LFD and then do another comparison.  Just upgrading to a better DAC should help quite a bit. 

adg

It's so good I no longer use my Audio Alchemy DPA-1 and PS5

Best ~$400 I spent. Way better than a cable or PC upgrade, which will still improve it

Keep in mind the foundation of the music begins with the PS

tweak1

I totally agree it all starts with the PS, and don’t at all question the IEC socket didn’t help things but I think I’d still look at a DAC instead of putting the money into an Oppo first. That being said it still might makes sense to upgrade the Oppo as a transport only if one chooses another DAC. I’ve modified equipment myself and always achieved better sound but I don’t think a power supply alone will make an Oppo sound like say a RME DAC or others, but maybe it will. I’ve not heard a modified ModWright/Oppo player but owning a ModWright preamp I have no doubt Dan doesn’t transform the Oppo’s but at a much steeper price. I’ve thought about changing out the PS in my UDP-205, so maybe I’ll try it and report back... If for anything I can change out the fuse then.