I just tortured myself finding the cause of the balance problem within my system. Previously this was not a problem so I was surprised that in my ridiculously expensive, I having the sound center of my stereo pair floating closer to the left main speaker instead of the center.
I used some perfectly centered test tones available online to first test my suspicion. At first you think you are just "hearing things". To complicate matters I have been suffering from a cold and figured one of my ears was clogged up. Steadily I realized it was not my ears. To make matters more interesting, I just switched from an Integra 80.3 to a Marantz 8801 and I was worried the new unit was faulty.
First step: checked my speaker level settings and manually made sure neither of the speakers was accidentally set to high or low. This was not the case.
Second step: I switched the left and right channel to my amplifier to see if any difference was present (such as skewing to the right). This was not the case.
I had recently been messing with my subwoofers and wiggling speakers around the room so:
Third Step: I re-centered EVERYTHING to the 1/4" to be sure my main listening position was lined up correctly. Despite doing this (my speakers were off a few inches), the problem persisted.
My room is acoustically treated and perfectly symmetrical so I knew the problem was not a boundary / room reflection problem, especially after setting up the location of the speakers perfectly
Eventually I discovered that the right speaker's banana plug had "flattened out" and lost its tension. Possibly during adjustments of my speakers, the wires came just slightly loose. I pulled off the banana connectors, used some pliers to widen them out (so they have more tension when they are placed into the speaker receptacle).
The problem was SOLVED! Thank god nothing was wrong with my speakers, amplifier, or pre-amp.... that's always a much worse nightmare.
So in summary: for a balance problem: check with 100% certainty that your wiring has PERFECT connections. A partial connection can generate sound but increase resistance substantially and affect your balance.