I don't think that either mic has a filter on it... Does the filter just slide on top of the existing grid - making the mic a few mm longer?
As for the Ebay/PayPal route, looks like I've missed the deadline for claiming that an item is faulty/"significantly not as described". Even if I'd discovered the fault earlier, I reckon snipping off the Lemos and soldering on a miniplug would have left me on shaky ground.
A strange thing is that when I've 'tested' the mics by recording both acoustic and electric guitars at home, there doesn't seem to be any significant difference in response.
Another possible factor to be considered is that when I opened the battery box after taping the other night, I found that the 9v cell hadn't been clipped into the contacts properly, meaning that contact wasn't great and could have been lost intermittently. But I'm guessing that this can't account for a lack of bass on one channel.
I've not tried re-soldering the connections yet. I wasn't confident of my abilities to do the original job myself so I took the mics into a local PA shop who said it'd cost "almost nothing". When they'd finished (two-and-a-half weeks later!), they charged me £20 (c$40) - money I can ill afford to splash around. I gave them a bell and described the problem to them but the guy wasn't terribly helpful. He said he'd check the joints for me. Brilliant. I don't particularly want to ask them to reverse how the mics are wired only to find that the problem is with the mic and then have to foot another $20 bill.
Again, any further thoughts on what might be going on here would be very much appreciated.
Yousef