2-channel listening with a sub-woofer


Is anyone using a subwoofer for 2-channel listening? Have you ever heard a subwoofer for 2-channel listening?

If so, did the sub integrate seamlessly with the speakers? What sub was it?

When using a subwoofer, is it no longer important for the 2-channel amp to be able to drive the woofers?

For 2-channel, is a sub with a 15 inch driver faster than one with an 18 inch driver?

(say a 15 inch Velodyne DD vs an 18 inch Velodyne DD.
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I'm running a Rel Storm III with Totem Mani-2's and it integrates perfectly. I previously ran the Rel with Totem Sttaf's (prior to upgrading to Mani) and it was also a beautiful combination. The Mani's have much deeper bass than Sttafs so the sub is less necessary. Finally I also had a Rel Storm I set up with GMA Europas. A sub with the Europas is essential IMO.

My point is all 3 above setups were stereo, integration IMO was perfect and somewhat simple (though somewhat harder to set up with the Europas). For the Europas the x-over was at 49Hz, Sttafs at 38Hz, Mani-2s at 28Hz. In all 3 cases the soundstage is bigger, imaging at least as good.
My first sub-satelite was the MUSE sub with the TOTEM Model 1. Worked great, bought the personality card (custom crossover made for the totem model 1) but the transformer had a hum. It drove me nuts. Fast forward to my present set up and the sub integrates really well. You have to plot on paper with the use of a sound pressure meter and a controllable source eg cd player with a test cd such as Stereophile,Wilson etc. Then you play for hours or days to try to get rid of any valley or peaks in your chart. If you want more help E-mail me your phone number.
I have heard a lot about how hard it is to "integrate" a subwoofer with planar speakers like my Maggies. In my custom designed subwoofer system,I have been very successful in this regard, and I think that the reasons are as follows.

1...I went with two large drivers (12" and 15") so that the subwoofer operates with large area and small excursion, like the Maggies. (A line array of eight 8" would probably have worked just as well).

2...I located the SW systems (I have three) in the wall directly behind the Maggies. I describe this setup as the SW "playing through" the screens. It is as if all the sound is coming from the screens. Also, I think that the setup may involve the SW "absorbing" the backwave of the Maggies. Whatever...it works.

When I had box speakers I used SW enclosures as stands. This makes sense to me because it just amounts to adding the LF capable driver which the speaker manufacturer left out, and the enclosure happens to come in two pieces.
I have CLS's and also use a single Depth sub. I've found that the best integration is achieved by running your mains full range and fleshing the bass in from the bottom. Getting the subs bass level set correctly is the biggest challenge. Most set this level way to high. I use a Behringer DSP8024 as an RTA to help adjust levels and check room responce. ($150 w/mic on Ebay)
I am using a REL Strata III with a pair of Dynaudio Focus 140's. They integrate very well.