Driver matching vs break in period

I installed a new mid range driver for my B&W 803 SII and noticed that the image shifted to the speaker with the older driver. I got out my radio shack SLP meter and found that the new mid range driver was about 2-3 DB lower than the old driver in the other speaker. I ended up reinstalling the old driver and putting the new one back in the box. The question is: Is this a matter of matching drivers, or is a matter of allowing the new driver to break in, or is a bad driver? Any thoughts?
That is odd. Normally a 2 or 3 db shift should not move the image. Are you sure it is exactly the same driver - something else may be wrong.

The image is based on the time arrival of the primary sound at the ears - it takes a LOT of volume level difference to overcome this - probably as much as 10 db would be necessary to begin to shift the image. (Haas effect)
Most reputable manufacturers measure and match drivers (or match output by tweeking crossovers) for either speaker pairs or to match a model's reference (or output level) closely since any miss-match does affect imaging. I also believe that break in can increase a drivers output but may be only by 1 db or so.
Did they have the same part number?Also switch them (Midrange drivers)between channels to see if crossover is ok.
I guess I should have said,try the new driver in the good speaker and see if it still has a big loss. I don't think that kind of loss is normal.Make sure the plus and minus are correct also.That my 2ยข
Might be a crossover problem
I did switch the "new quiet" driver to the other cabinet and it was still "quieter", so it was not the crossover. I also might have over emphasized image shifting issue, while the image did shift a it was the additional "loudness" that was the main issue. at times, it was almost like having one speaker in the room, especially on vocals. My guess is that the driver might have been well matched with my cross over. Maybe I'll email the B&W representative and ask them what the issue is.
That sounds like a good move.I've had hard,or a little brighter on a couple mids in the past.On woofers,I've had boominess until it broke in.I can't ever remember a quieter one.At least they will know of the problem so you don't get stuck with an unusable driver.
Here is B&W's response:

"The speakers are all calibrated to a specific spec. Please check the capacitors and resistors on your crossover network. Too often I have seen this open."

Maybe I'll check the crossover values, but I'm not sure why it would be the cross over when the problems follows the driver when I change it to the other speaker.
It sounds like a typical I don't want to be bothered answer.Just give me my paycheck.They used to work with a customer from what I remember.
To check the crossover, do I need to actually get to the resistors, capacitors, etc? Or can I measure the value at the speaker leads and compare it with a new crossover?
To test each component, you need a LCR meter and each component should be removed from the circuit. Before doing that, I would try contacting them again and hope someone else receives your message.