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. 

It sounds like an out of phase problem. Reverse the positive and negative on the speaker and see if that fixes the problem. It’s a shot in the dark but you have nothin’ to lose givin’ that a try!!

Presumably you have listened to the tweeter, putting your hand over the mid-bass unit that follows jsut to check? Likewise with the top mid-bass unit?

This might help you single out which unit has lower amplitude.

Otherwise (assuming it's not just a phase issue as mentioned above), you'll need the help of a technician. It may just be a bad soldering

Use an SPL meter and measure mono signals.  Measure close to the drivers and do it at relatively low volumes, say 70db.

Thanks for all the good suggestions.

Yes, I have covered the speakers to determine if they are working but have not measured with an SPL meter. Will investigate the latter possibility.

I don't think speaker placement is the culprit as the problem moves with the speaker position, i.e. the loud speaker is louder in either position.

I will try the out of phase speaker wire reversal, but f I haven't experienced this with other speakers, then I assume the out of phase miswiring would be internal to the speaker itself. 

Unfortunately I don't have any balance controls on my integrated amp so I can't try this test comparing a mono signal.

I can measure speaker coil resistance and look at the crossover connections. I can also solder but delving into the crossover network and measuring/replacing individual components would be a last resort.

Will report back!