Installed Wrong Tube 12AX& instead of 12 AT7

I just got a MC275 yesterday and the seller mistakenly installed a 12AX7 into the V4 socket (originally for 12AT7) It didn't sound clipped or it didn't stop functioning except that one channel was louder than the other, but I really don't know if the mistake has damaged the amp, or the damage is slightly and I don't see or hear... I am curious to know if anyone has done the same and what happened... Also, what's most likely is to be damaged if this mistake occurs. What should I look out for... or how do I check. All your inputs will be greatly appreciated.
The only defect in a tube that can cause damage is an inter element short. These are common enough in power tubes where they usually take out a resistor and/or blow a fuse, but are rarely seen in small signal (dual) triodes like the 12AX7 or 12AT7.

You should be fine.
No matching required. Power tubes need to be matched (unless the amp supports individual bias adjustments per tube) to ensure that the tubes equally share the load. Otherwise one tube might hog most of the load, limit ultimate amp performance, and fail prematurely.

Small signal tubes are self biased, produce voltage gain only rather than power, aren't running parallel and/or push-pull, and don't need matching in normal amplifier topologies (including MC275).

Feel free to experiment with different tube brands. Tube rolling is part of the fun.
If you don't match the transconductance of the small signal tubes, you WILL have a channel imbalance(to the degree that the tubes' gains don't match).
Rodman, negative feedback (mandatory in a push-pull pentode amp) will offset 90 to 95% of the transcoductance delta.

You are 100% correct regarding 12a_7 tubes in zero-feedback SET amps. These must be matched if you want a central stereo image.