Confused about performance from SI18s

I am confused right now because when I add in my sub, a dual opposed SI 18, I test with REW to have more output. However when I turn off the sub, I feel the bass much stronger. My current HT is an unfinished basement. The total size of the basement is around 1400sqft, however I have my room tucked into a bump out that is 13 x 20. The back is wide open to the basement. I am surrounded on the front, sides, and floor with concrete.

My system consists of:
Onkyo TX-NR818
Mains DCM -KX12
Sub Dual Opposed SI 18 running off an EP4000
MiniDSP UMIK
Balanced MiniDSP

I have had my dual opposed 18s working for the last week. I like the way they sound, and according to my UMIK I can get over 120dB at 15Hz and up at my LP. I can probably go louder, that is just where I stopped testing. I am confused because I have DCM KX12 mains and as the name implies, they have 12" woofers. The speakers are very capable of putting large amounts of bass at high volume.

Anyway, when I add in my sub, I lose impact. I can tell that the system digs deeper, however I don't feel it anywhere as much through the couch. When I turn off the subs in my Onkyo TX-NR818 I have a lot of impact and it shakes the couch. With the sub turned on, the feeling of exposive bass disappears to a large degree.

I am a bit confused because it is obious with REW that I am getting more bass at every frequency with the addition of the sub. The thing I notice is that the main speakers without the sub produce impactful bass, however it doesn't feel as deep. When I add in the sub it sounds deeper.

I have tried crossing at various frequencies from 40 up to 150 and it doesn't bring back that feeling I get from just the mains. I have a feeling that the mains are playing the notes at a higher pitch than the sub, thus I feel it more.

When I look at the response that I posted, the sub plot is always above the speakers, for all frequencies. That is why I was confused that it wound sound louder, because technically it is louder with the sub added in.

I also thought I didn't have to worry about polarity because I upped my crossover to 150 or 200, however I just tested and i didn't realize that so much was still being output by the mains. I assumed that the mains wouldn't be playing very much in the 50-80 range so that I wouldn't have to worry about polarity and the mains cancelling the sub. I was wrong, the mains do still have output with the crossover so high, so polarity does matter.

I played a constant 60Hz sine wave and then inverted the sub signal, it did get a fair amount louder. I gained 4.5 db by inverting at 60Hz. At 70 I have gained 6.5db.

I do like the subs, they put out a lot of pressure and mine hardly even move. I have 2 more that I think I plan on building into single boxes, but I am in no hurry because this puts out more vol than I need.

I tried a few sweeps with it inverted and not inverted. I gain from volume below 80hz if I invert, however I lose vol above 80 when inverted...

I should note that I reset the miniDSP before tonights testing/tuning, so I am not boosting or cutting anything.

I know that I can boost my signal to make it get the impact, I just don't understand why I don't have the impact when the mic says that adding the sub gains me SPL at pretty much every frequency. It seems strange to me.

I am using audyssey so I guess I do have an eq. My room may or may not have that much room gain. My sub is centered nearly touching the front wall, firing left and right.

I didn't really phrase my question well, I wasn't worried about the sub not performing. I was just confused how I can use a calibrated mic that shows more ouput with a sub, but it felt/sounded louder without a sub.

I realized that I was testing in Stereo mode using REW, however the test clips I was listening to are Dolby Digital. I reencoded my test clips into stereo from 5.1 so that I can now listen to my test tracks in stereo, the same as I am measuing. The phenomenom is gone in stereo mode, adding the sub only makes it sound better with more impact. I believe what was happening is that without the sub, the LFE track was added into the mains and for some reason, they ended up very "hot" and boomy/impactful. In stereo mode, there is no LFE track to add into the mains, so the sub only makes it feel better.

I'm not sure why the Onkyo TX-NR818 made the mains that much more powerful, but it doesn't matter to me. I know now that I am getting more output AND feel from the sub, at least in stereo mode. I'm going to assume that the extra bass in the mains without the sub is wrong for now.

I did notice something strange with my receiver. When I turn off the sub, the noise floor gets considerably louder in the mains. With the sub turned on in the receiver, the noise from the speakers gets much quieter.
Edited by NathanJ - 1/20/13 at 2:59pm