I don't think it's hugely steep. I've just been experimenting with Low EQ on a B3n - thought I might as well eliminate at least some subsonics. Setting frequency to 40Hz on a 4-string and 20Hz on a 5-string makes no difference to the audible sound. I've put the Low EQ as the very first stage, before a compressor stage, so the compressor isn't getting phantom sounds to compress. I did some experimenting with changing the frequency to see what the effect was, and there didn't seem to be a huge roll-off as I increased the frequency. It may be relevant that I'm doing this through a Line 6 G50 wireless to avoid having to disturb my pedalboard, and that might have some sort of HPF in it.
I really ought to get round to putting together a Sallen-Key filter.