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?
I’d also like to get anyone’s input on 12ax7 tube replacements.  My budget would be $200-$250 per tube.
The NOS tubes in my phono stage have been sounding great for nine years now, so if it is your Mullards that seems short lived or a defect. Are the Mullards supposed to be NOS? Is it coming from both channels? Both tubes, both channels, same time, does not sound right. If one channel then switch the tubes and see if your distortion has moved.
Todd, as Kenny pointed out a little judicious tube swapping can lead you to a bad tube if the distortion is channel-specific.  You don't mention what "way up" might mean but 15 hrs/week for 1.5 years puts you in the 1,200 hour range.  Unless you have a preamp that is know to run tubes exceptionally hard (Audible Illusions comes to mind), preamp tubes can usually be counted on for 5,000-10,000 hours of life.  Dick
Shuguang Treasure12AX7 LS from Grant Fidelity.
Give then 20 hours to break in. They best my Telefunkens.
@kennythekey @djohnson54 thank you for the input.  I should have qualified a little more...

”Way up”, they probably ran for about 4-6/day for the first year, due to the preamp being inband of the home theater signal.  I’ll swap the L/R channels, but it sounded more as if it was coming from the L channel; which could be due to the pan/mix.  I bypassed my preamp all together and am running it direct from my DEQX unit and it’s DAC.  I use a McIntosh C2500, I don’t know if those run super hot or not, per your comment @djohnson54 .  Running it through the DEQX, the harshness is removed.  Faulty tube, very well could be.  I probably have about 2.5k - 3.5k hours on them.

@kennythekey They are NOS.  I bought them from Kevin at Upscale Audio.  They’re the Mullard CV4004 12AX7 NOS.

@noromance , I’ll check them out.  Do you have any input on their musical characteristics?
A few years ago I thought I had a tube issue in my pre-amp.  Turns out the volume control pot was dirty, and the sound cleared right up after I sprayed some Caig De-Oxit into it.
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. 

All, thank you for the inputs.