^^^ What Chris is suggesting is that you should never use bass roll off in the field, and always do it in post. I found personally that with some mics, to make a pleasing recording, I almost always needed a touch of rolloff, and the rolloff provided by a battery box with it built in would do the trick. But, I got to that point after a lot of practice. Doing what Chris says will not go wrong.
And the more important point he makes, I think, is that if your recordings have *distortion*, using bass rolloff will not eliminate that distortion, nor will it ultimately improve the response of your mic. The only way to make recordings of loud concerts without distortion is to start with a mic that can tolerate high sound pressure levels. Chris' product is designed specifically to do that.