One speaker is louder than the other

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.

Are you sure both drivers in the low volume speaker are functioning (ear next to driver with low volume music. I had a similar problem and one tweeter was blown.

I had that problem before I moved and then it disappeared when I moved to a different house. Besides the change in rooms, the cables were switched during the move. Have you tried switching the cables? Stranger things have happened in this crazy hobby.

switch loudspeakers from left to right see if problems follows loudspeaker if so its in the speaker not the system.