Larry, JohnB. and Rick are correct; the extra set of sub outputs on the 3480 are simply a duplicate set of full-range line-level outputs and have no crossover at 100Hz or any other frequency. Sometimes you have to read between the lines in the manual to determine this and sometimes, like this, there's nothing even between the lines. If you want to satisfy yourself that this is the case, it's easy to do. Disconnect the sub cable from the sub; remove the jumpers on the back of the 3480 that connect its amp sections to the pre-outs; connect the sub cable to one of the main inputs. You'll then hear that main speaker playing a full-range mono output, not one sharply rolled off above 100Hz. If it was just 100Hz there would just be low thuds; despite the use of the term, subs in and of themselves can't be "musical". It's the combination of good subs playing the the 20-80Hz fundamental frequencies well and good speakers playing the upper harmonics to the fundamental frequencies that make a low frequency note sound "musical".
You may not have your setup adjusted to your best advantage yet. Although it's possible to buy separate filters(e.g. Harrisons from Parts Express)to handle bass management, with what you have now the filter on your S-8 should be switched in so that you can roll its response off. Since, as JohnB. mentioned, the M22s will be rolling off below about 60Hz, the S-8 crossover should be set for that same frequency area. Note that the 40-120Hz markings may not be very accurate, but nevertheless a setting at or just below the 60Hz mark may give good results.
