Midrange Increasingly Harsh

FWIW, I've noticed some distortion in my PL amp. I changed out  the power tubes - different tone but distortion still there. Short version, I finally just put in new drivers because I had back up ones of the same type/brand, etc, because I like their sound. Bingo! Everything's fine now a the cost of just two 12AX7's. 

@jrcotner 

This is an extremely common problem call sibilance. The problem is that our ears are most sensitive at frequencies in the 3 kHz to 4 kHz region exactly the same frequencies reflections in most rooms are loudest. It is more common with dynamic speakers with broader radiation characteristics. Horns and planar dipoles are not so affected because they are directive.  The solutions are use an EQ curve with a Gundry dip in it. Or get down with some room treatment. You need to put absorption at all first reflection points walls ceilings and floors. Or you could get yourself a set of horn or planar speakers but you might still require some room treatment although it should be a lot less. I Use 8 foot tall ESLs 36" wide. The only room treatment I needed was three rows of 4" foam acoustic tile directly behind the speakers. Still the occasional recording will cause some sibilance due to the way they were mixed. 

It remains unclear as to what might be going on here.  First, the OP reported a developing problem—one that was not present at first, but came to be and has gotten worse.  If this is true, and is not a hearing problem, then it is something in the equipment that is going bad.  If it is equipment going bad, the prime suspects would be the tubes or the ferrofluid in the tweeters drying out.

But, it could also the case that the OP did not notice this issue at first, but now that he is aware of it, it seems to be getting worse because he is focused on it.  In that case, suggested cures by room treatment or equalization may make sense.  Again, I would suggest experimenting with nearfield listening to reduce the influence of the room on the sound to see if the room is the primary source of the problem.  If nearfield listening cures the problem, but is not otherwise practical, one would still be informed as to room problems that can be attacked with treatment or DSP compensation or equalization, change in speaker or listening chair position, etc.

The improvement when using the 4 ohm tap is an interesting clue.  That tap has a reduced source impedance (the output impedance of the amp) and this reduces the frequency response changes attributable to the interaction with the speaker’s changing impedance at different frequencies.  This, as mijostyn suggests, might be a problem associated with a boost at certain frequencies.  The “cure” might be equalization or even using a solid state amp because such amps inherently have low output impedance.

That you changed speaker wires to the 4 ohm taps on your amp b4 replacing the tubes and heard a slight improvement does show component issue between speakers and amp and replacing the tubes should return the sound to what you're used to. ferrofluid drying up in tweeters is also very real and likely to affect sound before xover components wearing out. Listen to me acting like I know what I'm talking about, lol. You've been given a good lead to get your tweeters serviced and it might be worth it to you to be without your stereo the week or two just to have them tip top. Either way, good luck.