For HT, how important is extension south of 20 Hz?

I think it is important for the sub to go below 20, especially for movies, you can't hear it, but you can feel it. Sometimes it feels like my couch is shaking. As Bulldogger said: "The bass down that low causes your clothing to move " totally agree with that.

I recently completed building (2) sonosubs using a design found over on AVSforum with TCsounds TC-2000 15" drivers in each sub. They are flat to 14hz, am driving them with 1000 watts to each sub. This is my first experience with bass of this type for HT. It is just plain fun watching the expressions on my friend's faces when they are over watching a movie or even some concert DVD's (like PF Pulse). Some movies are just plain scary. Like Dave said above, room pressurization is awesome. My recliner feels like one of those electric massage chairs. Supposedly, there is a fair amount of LFE info in the 6-10hz range.

My current subwoofer system is based upon (4)15" Adire Tumults.They are sealed.Two per cabinet in separate chambers.

I use (4) QSC 1450's, bridged mono to each Tumult.They have a relative high F3 (around 38hz in a 3.0 cu. ft. enclosure).To counter that rolloff I use a Marchand BASSIS to extend the bass down into the 5-8hz range.My Behringer 1124p is used for one bad room mode.

WTS, deep bass really fills the frequency response.There is information down into 8hz that needs to be felt not heard.There is an energized feel when the room pressurizes.It's almost like surround bass.In WOTW the lightning strikes are around 8-10hz and at 90db it hits home and gives you that feel that a 20hz high pass does not.

Make no mistake getting down that low is a DIY job if you don't want to go broke.I don't know of many subs capable on the commercial market that can get that low without a mortgage attached, if at all. I'm not sure the return is there for you unless you feel it for yourself.it's a personal thing.If I'm going to buy a DVD I want all the sound....even down to single digits.

Enjoy..hope this helps KG

I use two Martin Logan Decents. I think they go down to 18Hz. I've spoken with you a bunch of times over the phone but we have never met. You gave me advice about Mcintosh monoblock, MC1000s with the Soundlabs. I think we spoke last a month or so after the hurricane.