well normally attenuator cables drop a hot incoming signal so it doesn't overload the recording/recorder. However, I'd imagine since the battery box is boosting the signal and the MT is boosting the signal as well, dropping it down before it gets to the MT will allow you to use the preamp in the MT more efficiently. That will allow you to set your levels better on the MT so you don't clip, or have the fear of clipping.
I'd imagine if you lower the game on the FET, that'd pretty much have the same effect. Since theres no way to lower the MT, I think the only option is to lower it before it gets to the MT. You could always try running mics > mt and see if that helps it out at all.