I wouldn't think it would be an issue of cap failure either, though perhaps it is just the characteristic of the mics or capsules themselves (i.e., they're working fine, they just don't provide much bass response). Not saying that is the case since I haven't used the Busman mics or the subcard capsules. I've owned several pairs of Chinese-sourced mics that I've found to be thin sounding though, without the bass response I prefer.
Also, when it comes to anemic bass response, you should double-check that both channels have the same polarity (are in phase with one another). Zoom way into the waveform using your DAW and look at a single transient. Make sure that when one channel goes positive, the other goes positive with it and is not mirrored to the negative side.