One thing I don't see discussed here at all (tho I haven't read them ALL), is something Jim Fosgate did with his Dolby Pro Logic-II invention, and moreso, the improvements he did to it after DPL-II. Screw the quad records - he was happy with what he did with the Tate 101-A (and his later tweaks). Screw the fact that DPL-II decoded ac3 files with 39 dB of separation. He was happy with that. But What he did at his many home demos that he hosted over the years was to listen to STEREO records and listen to what DPl-II did to THOSE. He made dozens and dozens of mix CD's of stereo stuff that worked really well, using DPL-II. The more "ambient, the better". I swear if he wasn't inventing stuff (which he always was, up to his death-the last one came out 2 weeks after he passed), he was making mix CD's-- Of STEREO content, to be played at his demos. I have dozennnns of them. He could have cared less about how DPL-II decoded DPL-II files anymore, what he wanted to show the world was "USE IT ON YOUR STEREO RECORDINGS!"
And THAT is why I suggest keeping your 5.1 DPL-II alive. It's why I do. Expand your stereo stuff to 5.1 (to 7.1 if you're lucky enuff to have a FAP-1 in 7.1-I have 3). It does things that can be quite shocking. Like a clarinet in the rear right that from a Bruce Swedien record, seeems like it was mixed that way. An album I MADE for a blues-rock band decodes with 1 guitar in the rear left. I didn't mix it that way... but it's only in the rear left.
Look, Regardless of the quality of your system, enjoy what is does to stereo. I teach at a university in Digital Media (Audio, duh) and I encourage my students to use that system to make albeit "fake" surrounds, definitely a form of spatial audio that you don't get from your lazy-ass stereo mixes (yes I encourage them to mix in surround and atmos, but how many take me up on that? Yes, you are correct, 3%).
Yeah, I record in surround alot, and you can move alot of your surround (and stereo) stuff up to dolby atmos. But what modern formats, which can render your 7.1 files or 7.2.4 from a bluray or game, into your speaker set, without loss, but they DO NOT upconvert the files like DPL-II does. Like it or not, it does enhance stereo. That statement comes from the inventor, and from me as well as. One thing Jim taught me that stuck.
-MikeWiz
If you like what the upmixing does, then use it. I have had good and bad experiences with it. Movies trend more towards upmixing being beneficial for me, music the opposite. If I engaged any modes on the Marantz that forced upmixing several 5.1 recordings would get the vocals spread out in an unappealing fashion and the dimensionality of the mix would be degraded. Stereo might be a bit better, but the L/R in my setup do a very good job with 2ch so I tend to leave it be.
As far as the DPL debate, the current flavor of Dolby upmixing built into the Lyngdorf seems to work fine for the occasional old recording played through it. Though I couldn't say how faithful it would be to an old school DPL system.