Proac Response 3.8 stereo imaging is not centered

You can try experimenting with speaker placement.  Even a small difference in location, toe-in etc. can affect imaging.  If you put the speakers at exactly the same spot as your previous pair, the new speakers and old may interact differently with the room and this might be at least part of the problem.  
 
Have you tried listening very carefully to both speakers to confirm that all drivers are working and sounding the same aside from the volume discrepancy?  Put on a mono recording or switch to mono mode to get an identical signal to both channels fot this test.  Now swing balance all the way to one side to the other to confirm that the speakers sound the same.  I am betting that they don’t sound the same.  If any component—a driver, a part in the crossover is off, the sound would change and not just the overall volume. 

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.  
 

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.

This is a very good speaker that is very much worth fixing.  A factory supplied driver is the best approach.  Even if an off-the-shelf ScanSpeak driver would work, you will have some lingering doubts that might hurt your ability to enjoy this fine epeaker.