Help troubleshooting: 1 speaker is playing significantly lower

You might simply switch the speaker cables and make right --- the left and legt ___right. 
Rules out the amp and the source. 

Start at one end of the system, like the speakers, and switch the wiring left and right.  If it is the same speaker that is softer, it is the speaker that is at fault (not likely).Next switch the wiring from the amp to the speaker wire (to rule out the speaker wire itself).  Next whatever wires are feeding the amp. . . all the way back, if necessary, to the source components.

At least that tells you where the problem lies, even if does not tell you what the problem is.  I recently had the same issue and went through swapping tubes left and right in amps and linestages, and it appeared to be in my linestage.  I was about to take it in for a check up, but, when I disconnected the interconnect between the linestage and my amp, I decided to try plugging it in to see what happens.  It turns out that this act cured the imbalance; the problem was probably less than great contact between the RCA plug on my interconnect and the jack on the back of my linestage.

@larryi Exactly the same thing that happened to me. It retaught me the lesson of checking all connections first, unplugging and plugging all, then going through the checklist you mentioned.