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.