Midrange Increasingly Harsh

As Elliot recommended try an isolate the problem. Most likely its either the speakers or the tubes since you’ve replicated the problem with multiple sources. You’ll need either an alternative amplifier or alternative set of speakers and see if you can replicate the problem after replacing one of them. Also is this coming from both speakers or just one side?

When it comes to tubes its more about hours played then age, power tubes can last around 2500 hrs and preamp tubes about double that, but like any piece of electronic gear, they can fail at any time, if you have single tube or single driver fail the problem would more likely be in one side. If it’s gradual decay over time its would be on both sides. So first see if its the amp or the speakers, then go from there.

Yep, I’d do a full re-tube. Your power tubes are definitely shot and the pre-amp tubes are at the end of their life. Try to avoid using it until you get the power tubes replaced at minimum as the distortion from spent tubes can damage the speakers. Fortunately you don’t play at high volumes so hopefully the drivers are unscathed.

Congrats on averaging 1k hours a year of listening! That’s tremendous. I work from home and even then I doubt I’m half that.  I think I'll do some listening now and try and catch up lol.