I think it's an EQ, not so much a pad. The mic would have to handle high SPL's on its own, which is why the ECM8000 might not be a good choice.
I assume that, presented with a relatively flat frequency response across the audio spectrum from a mic (or whatever comes out of the SR25 - I can't find a freq plot), that it equalizes the sound by attenuation of certain frequencies (the way any passive LCR EQ works) in a manner that makes for a pleasing kick sound.
Again, by speculation on my part, if presented with the frequency response of colored mics which already boost and attenuate the spectrum from a flatline, the kickpad would process this signal in the same manner, but the mic's own intrinsic deviations would influence the output also, and a "different" sound would be present. Whether this would be more or less preferable to the sound from the SR25 would be in the ear of the listener.