One speaker is louder than the other

I just got a pair of Celestion SL6si speakers from ebay. they sound wonderful, however one speaker is not as loud as the other. I need to adjust the gain on one speaker 2 clicks on my volume control

what would cause this? a Capacitor or resistor?

Have you tried other speakers to see if it's your preamp/amp? It could be a bad tube, or maybe some pots that need cleaned.
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 would contact the mfr for service.
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.
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.
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.
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.
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.
I think I have found the problem. the surrounds on both speakers are cracking and is worse on the right side. Plan to get them repaired this week.
"...I think I have found the problem. the surrounds on both speakers are cracking and is worse on the right side. Plan to get them repaired this week....."

That will do it. Let us know what happens after the repair.