I had this same issue with a new pair of DIY single driver speakers. Break-in did help some but not enough. Repositioning speakers did nothing. Switching to warmer tubes did very little but some. Switching from 4 ohm to 8 ohm speaker connections along with using Duelund DCA10Ga cables was my solution and happy ending. Not sure any of this applies but just sharing what I went through.
Speakers sound too bright.
I just bought a new pair of Martin Logan 60xti speakers. They are too bright and fatiguing. I would like to avoid returning them. I've tried toeing them in and out. I cant get them further than 1ft away from the wall (back of speaker to wall). I have a about 1-2 hrs of play time on them. Not sure if break in will help settle the upper frequencies down. Any suggestions...?