Proac Response 3.8 stereo imaging is not centered

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!

I think @yogiboy above, is referring to relative phase, not absolute phase. As in, one speaker is wired with the negative cable going to positive terminal, while the other is correctly. 

This will cause the image to be offset, or diffuse, and not focused. 

The test with an SPL meter is good. There are many decent ones you can download on your phone. 

OK, changing the phase had no discernable impact. HOWEVER, it appears the midbass driver is not making noise on the weak side DOH! It was moving with the music (maybe sympathetically given the sound waves inside the enclosure?) and the bass and tweeter were filling in around it but it is noticeably silent.

I checked the connections, cone, surround and spider. All appeared good. I pulled the crossover out and all solder connections were good and couldn't detect any loose parts. Unfortunately everything internal to the speaker from the driver connections to the board connections are soldered.

What is my next step? Unsolder everything and check component values? I can't check speaker resistance as long as it is soldered into the circuit. If everything is intact in the driver assembly can the driver still be faulty? A search online yields no used replacement drivers, but apparently a Scan-Speak 8545-01 is an "improved" replacement. Anyone have any experience with this mid driver replacement in the 3.8?