Proac Response 3.8 stereo imaging is not centered

You should check the capacitors in the crossover, they are the most likely thing to fail apart from the driver itself.  These have to be removed from the circuit to do an actual test.  If you have an ESR meter (equivalent series resistance) you can do an in circuit test that will give you a rough idea if the caps are good.  I know a lot of techs who think an ESR meter is more useful than a capacitance meter, and it is a lot cheaper too.

You can get a 9 volt battery and connect it to the tabs on that midbass driver to see if it moves in or out; if it does, you know the driver is not completely dead.  
 

I contacted the Sound Organization (Proac dealer) who will find out if the original drivers are still available. I'd like to stay original but $800 is a lot of money for one driver! I read the same thing about the Proac drivers being "modified" Scan Speaks but I have my doubts. Guess I'll pull the driver out tomorrow and unsolder it to perform the 9V test and go from there. If the caps on the crossover are electrolytics then they should be replaced anyways, so maybe a crossover overhaul is in my future. Fortunately the PC board is large and simple and the components are discrete and easily removable; the most uncrowded crossover I have ever seen. The components are pretty good quality for a 25 yr old design, with large gauge iron core inductors. Can't tell what caps were used, they appear to be Proac branded. I have requested a schematic form Proac to assist in identifying nominal values.

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.