1. That's hard for anyone to know. You would have to try it and find out. A peak at low frequencies tends to overwhelm the sound at that frequency. You may be hearing 40Hz and be missing information at 30Hz.
2. The low frequencies from a subwoofer are equalizable. The frequencies above that come from the mains are best dealt with using other methods such as room treatment.
3. No, you don't use an equalizer on the mains. It affects the subwoofer only.
4.
BFD 1124 or FBQ 2496. They're available at music stores, or on-line, or e-bay. See the
BFD Guide on using the BFD and
REW software for software to set it up.
brucek