To @bluethinker sure, no worries. Yes you have the correct understanding of what might be occurring here. For your awareness - this is really no different than many of us have experienced over many different speakers, amps, tube, cable, source, and or room changes too.
Where you may have gotten off course was the former output tubes wearing down, and you went out and bought the re-issue Mullard’s. I had the same thing occur with my preamp a few years back, I wanted to spare my vintage 6SN7 quad of tubes, put in the new Mullard’s in and while it had nice warm body, I lost some of the detail and zing on top. Some of these tubes are designed this way on purpose.
I did not want to mention further sending you down a Rabbit hole, but the small signal vintage Telefunken like @marco1 mentioned is one trick or the GL KT77s (more open and detailed), and some others like basic JJ EL34s are more detailed but have less body etc. Its just a balancing act. This is why I mentioned the Ray tubes, (not public, ime they are upper line tested PSVanes) relabeled and sold as Ray, and some folks in the headphone seen are commenting they have a little more air/upper detail with a nice balance, still kind of neutral. Its really as simple as you might be imagining. I dont know if the new PSVane Horizon is more neutral or not in an EL34. That might be worth asking on the CJ forum if anyone has tried them yet. Tubes can sound differently in different amp brands / models / circuits as you may know.
A few of us replied to "wait" and "not buy speakers cables yet" because we know that changing upstream tubes or amp tubes or maybe you could borrow (demo) another DAC from someone real quick to test that too. Sounds like a lot of it tilted you to the warm direction. Maybe one change at one or two of these upstream can tilt it back to more open, detailed, airy, and dynamic. We all go through this when we started - pairing up a bunch of components not originally designed to work together.
So we go back and fine tine up/down the stack here and there til it balances out where we want the sound to go - and so forth.