My tube amp sounds darker now.

I'm using Siemens CCAs, and after putting around 100 hours on the tubes, they are almost too dark sounding now. When I first got them, after about 20 hours of use I thought the sound was a hair too bright, so I bought warm cables to offset that, and everything was perfect for awhile. But now I think the sound is too dark, and I'm about to try some different cables again.
I noticed this just after I accidently left my amp on overnight (for 12 hours). This didn't damage the tube / amp did it? Ugh, what gives?