Subwoofer Crossover Setting/Directional Bass Freq.

I'm not sure that is the right sub for your application HT subs don't mate well with two channel only systems
Its more challenging to incorporate a single sub into a two channel system
Low bass under 80hz will be non directional as you stated
You will need to tweek with placement and crossover settings
You don't want the sub firing back at your main speakers from across the room. This will actually cancel out bass frequencies from the sub and speakers you end up with odd bass anomalies similar to the issue you are experiencing
The best place to start is to position the sub with your main speakers firing in the same direction.

My Veledyne DD-15 makes it very easy to equalize for the room and for a smooth integrated frequency curve with its software that you can see on the TV and then adjust.

I mainly listen to 2 channel but also some HT and my processor has both music and movie modes and the DD-15 is set-up differently for each mode.

My speaker's do down to 30-35hz with no problem but you do have to play with phase and polarity and crossover to get a seemless integration. One of the more common errors is setting the volume on the sub too high and 'hearing' the sub as a separate speaker. It should blend seemlessly into the music or movie sound without drawing attention to itself.

Quite honestly, I don't know how I would have been able to do all this without the Velodyne's room eq software and on-screen frequency graph that instantly depicts andy changes made to crossover, phase, polarity, placement, etc.

Without it as the previous poster said, I'd start maybe 5-10 hz above your speakers' low end range and go from there.

Be prepared to spend some time but once dialed in, it will sound very good even with stereo music.

I agree completely with Mr. Reynolds. I use a Velodyne HGS-15 with SMS-1 to supplement my KEF 104.2 mains. I find that the KEFs sound fine by themselves, but when used with the sub, they sound a bit more transparent if high-passed at 80 Hz. Maybe something about double coverage of the mid low range. Even careful listening cannot identify the sub as a separate source, except you realize the KEFs could not produce certain sounds, especially that felt more than heard vibration when a large pipe is invoked, the feeling you get at a live organ recital.

db

I agree with all that Bob has posted here, but I'd add that the single biggest
issue I have with augmentation is the difficulty in achieving smooth
integration. When "augmenting", you have a narrow range of
potential x-over frequencies that you're filling in "under". In this
case, let's say that means 30hz-40hz with your speakers in your room. If
there's lumpiness around or above those frequencies, I doubt that you'll ever
get good integration. My experience has been that the key to smooth
integration of subs is flat on-axis frequency response through the x-over
point.

That's why I tend to agree with Bob's comments about HT vs Music subs. A
really good sub designed primarily for HT will probably sound great if you
can get flat response through the x-over point, and even a so-so sub will
sound more than decent. I use decidedly so-so subs -by today's standards -
and, to my ear, they sound great because I've gotten them to integrate very
smoothly. OTOH, before I got them dialed in, they were a problem. My
system might sound even better if I used true high performance subs, but it
sounds awfully good right now.

Similarly, I suspect that even a great sub which is poorly integrated will drive
you nuts. IME, many rooms will cause too many peaks and nulls in full range
speakers to achieve flat enough response to avoid this problem. However,
even if you're fortunate enough to get the type of smooth bass response
from your main speakers that allow you to augment for flat response, there's
another issue.

Even in the absence of "lumpiness", there's the issue of level.
That is, even if your current speaker happens to be perfectly flat for 2 full
octaves above your chosen x-over point of, say, 35hz (possible, but highly
unlikely) how does that level compare with the rest of the frequency range
above? If the band from 35hz through 140hz (in this example) is perfectly
flat but elevated by 5db or 7db relative to the mids/highs (actually pretty
good results relative to many rooms I've had), you'll get good integration, but
probably too much bass overall.

If you can augment successfully, it may be a better choice, because you can
get the active x-over out of the chain. However, IMHO, replacement will
usually be a better choice because it gives you broader freedom to choose a
x-over point that is both smooth and appropriate in level. IME, though, it's
hard enough to get it right even WITH the flexibility offered by an active x-
over. ( That's why I also use EQ in the bass. )

Good Luck

Marty

My point was not to pick on HT subs but to say that a single HT sub integrated into a two channel system can impose additional problems if placement is less than ideal.
When using two subs in stereo there is more flexibility for tweaking each sub for its placement crossover and slope without allot of additional EQ. But then again the room size will need to be considered for the particular subs.
I've heard quite a few different HT subs in a two channel set up and they tend to be over equalized for two channel but do a great job in HT.
I just have not heard a HT sub go from HT to two channel seamlessly. Two channel bass is more challenging to harness than HT.

I also agree entirely with Bob Reynolds.
The only thing I would add is that in the 2)Bass Replacement option, not only will your existing speakers sound better because the woofers are not being asked to do what they can't, but your existing amplifiers will suddenly develop far more 'headroom' as they are relieved of the power-sapping chore of supplying frequencies below 80Hz?

Nice post Bob! I agree completely.