Just one other thing I just thought of, does the stereo mic have any gain switches on it? Is it set to '0' and not any negative value causing a low output?
Check, check and check.. The place where I work we sell these mic's so I have tried it on three 5d2's in addition, and also the highpass filter on and off, and also the gain to "0" and "-10". nothing helps. I tried filming myself talking with the tv on in the background, and at 30 cm with the 35 L , and no matter what settings, it's the exact same results. Also with two other mic's of the same kind.
My brothers have a production studio and produces music, and he too was stumped.
Reading Customer Reviews over at BH for this mic, it seems it's great for everything but DSLR's...