Midrange Increasingly Harsh

The first step is diagnostic—isolate the source of the problem before proceeding with random “solutions.”  I would start with figuring out if it is from both or one channel.  Preferably, use a mono recording where you hear a problem and, if you have a balance control, swing the balance from one side to the other and listen for a difference.  If you don’t have a balance control, try sitting much closer, and in the direct path of one speaker, then the other, to listen for differences.  The best approach would be to do switching at one end of the system until you find the culprit.  I suggest starting at the CD player.  When switching interconnects, it is best to have the system turned off or the volume all the way down.  Pull one channel and listen to just one channel.  Then do the same to listen to the other channel.  If the problem is in one channel, now switch the single channel left to right to see if the problem moves to a different channel; if it does, it is the CD player or its interconnect that is the source of the problem.  If it doesn’t it is something downstream.  If the problem is in both channels, it will be hard to definitely isolate the problem without trying other gear to see if the problem goes away.  If you have determined it is not the CD player, do the switching routine to determine if the problem is the amp or the speakers.  For this step, avoid playing with one speaker disconnected from the amp (tube amps don’t like this).  Hopefully you will hear any difference with both speakers playing,  If switching left and right speakers does cause the problem to switch channels, it is the speakers at fault.  

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.

I listen at lunch and again in the evening with my adult beverage.  I have post-concussion phonophobia, which I suspected at first.  But my wife also noticed it so I can rule that out.  I don't have any sources for spare gear around here to swap in and out.  What I may try is plugging the CD player into other output taps from the amp, and fooling around with some different cables.  By my estimate I have over 5000 hours on these tubes, so they need to get replaced in any event.  I don't recall the brand but the were made in England and sounded wonderful for a long time.  Hopefully Upscale has my purchase history if I can't run down the receipt.

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. 

Even if it is not the tubes getting worn, it is helpful to have a full complement of replacement tubes.  Any time you suspect tubes are going bad, you can then do substitutions to see if that cures the deterioration.  Even when you don’t hear an obvious problem, performance can decline gradually and you may not notice this until you do substitutions.