The bass management feature on most receivers (and all THX-certified ones) are "digitally accurate" at 24dB/oct on the low-pass side, and 12dB/oct on the high-pass side. The latter is set shallower so that when it combines with the natural acoustic bass roll-off of typical "small" speakers it will mimic a ~24dB/oct cutoff. In fact, THX-certified satellite speakers are consciously designed to give a 12dB/oct roll-off below 80Hz.
Anyway, if you use the digital bass management provided by the receiver, you don't want to have another low-pass filter to further affect these precisely designed roll-offs -- unless, as I said above, you know what you are doing and want to add some exotic tweaks.
