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.  

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.

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.