with the jb3, you must divide the mHz by 3.7 (the resisitance of the device) to get a better idea of how much roll is happening.
when i use my spsb-7, i generally use 69 - 107, depending on the type of music and the venue.
69 = 18.6
95 = 25.7
107 = 28.9
the effect is almost nill... however, i look at it as a way to allow more gain to be added with less distortion from the low end.