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.

So i put the Sonus Farbers back into the system and I reversed the speaker cables as you guys recommended. Result was the left speaker was still very low. Now being that my secondary speakers (VA Haydn’s) play perfectly fine, my components are obviously not the issue. 
I don’t know much of anything about speaker builds, could the crossover have crapped out?

Make certain that both sets of speaker cable lugs are tight on the "jumper" if you are not bi-wiring.

That's all I got.

DeKay

you will need to fix the crossover to fix the problem. Open it up and find out where the problem is. Its not rocket science

True. It could be that something has come unsoldered, or you might find evidence of a disconnection on the crossover board itself. Be bold and see what you can find.

@dekay 

DUDE!!! I owe you!! I got these heavy duty jumpers on the Farber’s..two of the spades were touching! They must have moved during my speaker swap-outs. I repositioned and cinched them down, gassed up the system and BINGO! Problem solved! Can’t tell you guys how relieved I am...thank you all for your help.

Good...

Glad it was a simple fix (without tearing apart the crossover and/or drilling holes in the speakers cabinets to "tune" them-;).

Experienced the same problem with a pair of Castle Isis speakers a long time ago (while experimenting with bi-wiring/jumpers et cetera) thus the suggestion.

Interesting power amp, which I've read easily betters their old 100.2 (that I liked a lot) and I'm a diehard tube fan.

DeKay

two of the spades were touching!

you could damage your amp doing that.

@dekay 
It’s perhaps overkill for what I need but I do love it, no complaints whatsoever. 
@kenjit 
Rookie move on my part, I should be spot checking every time I speaker swap. Keeping my fingers crossed that everything is 100% 

You owe us a drink...

@sounds_real_audio 

😉👍