Proac Response 3.8 stereo imaging is not centered

If you swap speakers and the problem moves, it’s likely the speaker. Crossover components don’t fail very often while a mechanical thing like a driver is far more likely to have an issue. You could go one more step and swap the effected driver only- take one of the working speaker and put in the non working one and see if its better. If it were me I’d find someone to recone both drivers (in both L/R ) and see what happens. (The reason for both is the visuals, the reconed driver may look slightly different (shinier, less dust) and this may drive some people nuts and convince them they sound different. If you recone both at the same time it's like two new drivers.

As long as you’re looking you might try to confirm that the driver in question is an authentic ProAc driver and not a generic replacement.  Does it have the same identification markings as the other drivers?

You could check resistors if one is failing or someone replaced it with wrong value the output could be lacking or too much could cause what you described. That’s the cheapest place to start then film caps. I’m sure your interconnects are all good and connections are good. I would look for bad solder joints in the crossover and binding post. 

Yes, if it is easy to do, the driver swap will confirm that is, or is not the issue.  I hope it is not because drivers can be expensive.  The ideal would be a driver from the manufacturer.  The original driver may be a custom job.  It is quite easy for even the smallest manufacturer with not much of its own R&D capability to specify and get drivers customized.  A local dealer in my area who makes some of his own speakers to sell in the shop, had woofers customized for his requirements.  He specified some of the basic design elements and performance parameters, and the manufacturer built prototypes.  As a retail shop, which builds a somewhat limited number of custom speakers, the minimum order of 40 appeared to be a challenge, but, about two years later, he is ready for another order of 40.  That was it.  Easy.  If ProAc says their driver has been customized, it probably is a genuine claim.

OK, driver removed. It is original with the ProAc logo on the chassis/basket. Measured across the terminals and got open circuit. 9V battery test is moot if I've got an open circuit voice coil. I confirmed I am able to purchase a factory replacement through ProAc. They claim it is different from the Scan Speak replacement because it has a different chassis. A Scan Speak replacement 18W-8545 is about $225, the factory replacement is $500. Think I will stick with factory to stay as close to original as possible. BTW there was a lot of information on line since the DIY'ers in the early 2000's spent a lot of effort trying to reverse engineer and replicate this speaker. Thank you to all who participated in the dicussion!