Fairly suddenly , over last several listenings - especially with the female background vocals on "California Dreaming" there a lack of detail, a muffled sound, and it sounds like much of the detail, punch and soundstage has disappeared on all sources.
I think we can rule out the subs. Lol! All sources, so rule out player and streamer. Amp and speakers, possible but very unlikely. Preamp tubes should last many years but in spite of that you are right, the odds-on favorite is the preamp.
I would look at whatever tubes it uses and order a few. Can't cost much.
If you're really cost-conscious, then if the problem is the same in both channels and you swap tubes to confirm that, then I would think rectifier tube. Anything beyond that and I'm running out of ideas.