I can understand your confusion
Rythmik offers some more control settings that aren't found on a lot of other subs. The FV12 isn't too bad though, here's the run down:
The extension frequency is just selecting where the low end filter is going to kick in. The damping setting is selecting how quickly that filter is going to slope off the bass.
If you want the deepest extension possible, you set the sub to 14Hz and low damping. This tells the sub not to start filtering the bass until 14Hz and slope it off as gradually as possible. Start with this setting.
Depending on your room, you might get a lot of "room gain" in the really deep bass. Or you might need as much headroom as you can get at 20-25Hz and not care about anything lower than that. That's where the other options come in. It's a similar idea. At the 20Hz setting, the sub activates a filter starting at 20Hz. The damping setting adjusts how steep the filter will be. So with a 20Hz setting and the "high" damping setting, you basically get a "brick wall" at 20Hz. The sub really won't play anything below 20Hz with such a setup - the slope of the filter will be very steep. But this gets you a little more headroom and output at 20Hz since the sub isn't even trying to ever play anything lower.
If you get a lot of that really low "room gain", you can use these settings to tame that. You might find that your room boosts everything starting at about 30Hz on down. In such a case, you might choose the 28Hz extension setting and then adjust the damping to get the right sort of slope to counteract your "room gain" and get a nice, flat frequency response again
To do that sort of thing right, you really need to be able to measure. You can get the
Room EQ Wizard (REW) software for free and simply plug an SPL meter into your computer to get some half-decent measurements. They won't be super accurate with just an SPL meter, but it's a good enough rough idea to at least get you in the ballpark. If you want something more accurate, you'll need a better microphone. For around $150, you can pick up the
Dayton EMM-6 microphone and the
Art Dual Pre USB that you'll need to connect that mic to your computer's USB port. It's about the least expensive measurement setup, but it gives pretty darn accurate readings. You can also get a
full measurement package from Dayton for about $300 that's got a USB mic and some really easy to use software if you're more comfortable with an "all-in-one" measurement solution
As for the phase and cross-over, leave those in their default "zero degree" phase and 12dB/octave-slash-External settings. Your receiver will handle those settings, so you don't need to adjust them on the sub itself.
Hope that helps!
