Distorted

Lately I have started to take notice of a slight distortion in "s" vocals, a little bit on "f" and "t" vocals aswell. I haven't had this problem before what I can recall and I haven't made any big changes in the setup. Moved around cables, changed heights, feet and placements of the tube pre amp and tube power amp. That's it. I have tried playing cd's directly from the cd analog outputs and using an external dac, no difference. Both speakers sound exactly the same so it's probably nothing wrong with them. Otherwise, the music sounds great, but this small thing is annoying me.

Any suggestions what could cause this problem?