Preamp tubes ran their life?

I believe I do know the answer, but I would like to hear inputs from the community.

Approximately 1.5 years ago I replaced the stock tubes in my preamp with some vintage Mullard UK ECC83’s.  I spend a fair amount of time listening to music.  I’d say, on average, anywhere between 10-15 hours a week.  For the first year I had them in, I was running my Home Theater with my 2-channel, via HT passthru; which would swing that 10-15 hours/week way up.   I liked the tubes and how they’ve sounded, pretty smooth throughout the band.  Today, I started noticing weird “slurring/distortion” in certain frequencies, especially with vocals and cymbal crashes.  I believe what I’m hearing is the tubes have ran their lifecycle.  Note: I do have room treatment and room/speaker correction and what I’m hearing just started happening this morning.

I won’t hold anyone to their words if it doesn’t end up being the tubes, but that’s my initial hunch.  Would your educated guess be the same?
The control pots from the old days (I.e., Dynaco, Eico, etc.) were known to get dusty and make noise. Yup, you had to spray them. I would not think your Mac would have that problem, but I'm not familiar with it. If you move the volume control back and forth do you hear a static like cracking?
I have a Mac C2500 preamp and the tubes should last 10,000 hours per the manufacturer.  However, I have found that the NOS Mullards can get noisy in a year or two time.
If they're starting to get noisy they're probably about done. Kind of a rushing slurry sound. By the time they make static they're getting close to failure I think.

Upscale I think still has British surplus Mullards. I bought two sets of the 12AT7s years ago for a VTA ST70. I'm still on the first set.

I agree, if you're using it for a home theater pass through you stack up the hours fast. I've gone through sets of 6922s in ARC preamps in a couple of years that way.
Todd, clear, clean, neutral, transparent, sweet and warm without being creamy or colored, great timbre, tonal color and speed. TBH, old Mullards are flabby and syrupy by comparison.