Also, using stereo modes, does stereo use the subwoofer output?

Mine doesn't. It just kicks in using 5 channel or more.
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Onkyo PR-SC886 (Retired Outlaw 970)
Outlaw 7100
Outlaw 200 Monoblock x2
M&K 851 for L/C/R
Axiom QS8 for SL/SR/1SB
SVS Pb12-ISD/2 (x2)
Sony VPL-HW15 (Retired Infocus 7205)
Sony PS3 / Pioneer DV-533