One speaker is louder than the other

You've swapped um so you know it isnt your amp. You've run a mono source so you know the inputs are equal. Your old speakers had the same volume in the same positions, so it isn't the room. Capacitor/resistor would be in xover I guess so is it part of the frequency range that's louder in one, or the whole spectrum? No buzzing in the softer speaker? Those are all the things I can think of.. good luck and hopefully someone can supply a definitive answer for you.

I have checked the other parts of my system, the problem is within 1 speaker. no buzzing. I am running them with a celestion sub with it own amp from 40 hz and below so the Speaker is ruuning essenitially full range.

I would contact the mfr for service.

Not knowing your setup, have to tried disconnecting the sub from the system. You would have to disconnect the cables from the preamp/integrated rather than simply turning the sub off. You didn't mention if the sub was in use with your previous arrangement.

just to clarify. I also have a pair of Celestion Sl600, which are essentially the same as a sl6si. when I run the SL600 with everything being the same, images are dead center, volume is 50/50. When I put the Sl6si in, the left side is louder than the right side.