i use that same mm one with my camcorder
works great
i also heard the sp one sucks, and it only goes to 6db
the radio shack and shure ones are basically the same as the sp one, you adjust the volume knob and it cuts the signal
not sure if there is a way to know exactly how much is being cut unless you have meters to read too and most likely with a camcorder you don't
i didn't pay 30 for my mm one, i paid 18 but the place is no longer around - cascade