Digital Room Correction vs Room Treatments

Did you try plugging the port on your sub? Just push some foam or a sock into it. Yes of course playing with the phase (timing) will change the curve, that’s the whole idea. Get a second sub.

Does your SVS sub not have the ability to adjust remotely from your phone? There is more than phase to adjust. You need to also vary the output SPL, Raise or lower the crossover frequency, Change the slope of crossover from 6dB/octave to 24dB/octave, vary the Q and room gain. If you change any of these settings then go back and adjust phase again, and so forth.

Bass Traps?  from my post above:  The bass traps absorb some of the low frequencies that build up particularly in corners but not exclusively. They help reduce the severity of room modes and standing waves.

It appears you did not comprehend my long post trying to explain all this so the best advice now is to learn from the masters. Look at articles from Earl Geddes, Floyd Toole and Todd Welti on multi-subs. Fussing around with DSP will not get you there.

The issue is I have very limited adjustments on my sub.  See pic below:

So I can adjust phase and level, but nothing else.

I was also having some issues getting REW to output to mains and sub at the same time.  So when I played with phase using sub output only, it didn’t change anything because I think it’s the phase interaction with the mains that will cause different responses to the FR curve.  Just changing phase if the sub is the only thing playing the frequency does nothing to change the FR curve.

You got a few issues going on. Think you have a couple nulls going on.

The bass energy spike 30-40 is huge, then that massive hole at 50-80. Some peaks in the middle, with treble dropping off.

This tells me you have a boomy setup, with bright mids, and low highs. DSP will not fix this, just adjust. Don't think you can room treatment out of this.

Your speakers are in the wrong spot, or your listening spot is totally off. Think you should move the speakers around. It looks like the walls are enhancing the low bass, nulling out the mid-bass, the room appears to "dark" already with the highs. Maybe remove a couple of panels, or move them around the room. 

Do you have any tones controls active? Turn them all off, if you are taking readings, then adjust them after the readings.

The 35Hz spike is there regardless of speaker placement and LP - varies a tiny bit in magnitude, but not much.  Room is 11.5’ wide by 15.5’ long and 8’ high.  Probably a characteristic of the room.

The 60Hz dip can be filled in at the expense of creating a more shallow one around 180Hz.

The speakers are places 1/5 of the room length from the front wall and 1/5 of room width from the side walls.  It was recommended to place the LP about 1/3 room length from back wall (60”), but I found I preferred my LP 54” from the back wall.

No tone controls active.  I’m using Pure Direct mode on my AVR to bypass the Audessey room correction.  I’m only using the AVR for the sweeps because it can accept the HDMI from my MacBook.  Normally, I use a McIntosh C35 preamp that has a 5-band EQ, but I normally listen flat.  Sometimes with a small bit of loudness contour, but mostly flat.

OP, let me apologise for assuming your SVS sub could do what I mentioned. Looks like you’ve got one of those  early models. I’m using an SVS SB 1000 PRO which sells for about $600 and has a very useful feature. Download the App and you can adjust all the parameters I mentioned from your phone but forgot there is also a few bands of PEQ. Check out their site. This is the sort of sub you need. There are other good subs available but the SVS is great value and I prefer it to my REL which has 0 or 180 degree choice so means I have to keep trying different places to get it dialed in.

I am using OmniMic to measure and I am not familiar with REW which  essentially does the same thing. I play a CD with sweep tones and have the mic. at my listening position sampling the combined sound of the speakers and sub/subs and while changing settings on your phone you can watch the frequency response change. The second sub will smooth the response even more. REW will also display  RT60 as a waterfall plot (CSD), the time it takes for the sound to decay by 60dB. The average room will be about 300ms.

I have 2 large floor to ceiling bass traps, broad-band absorbers, extra heavy curtains plus 2 subs and the bass is dynamic, very detailed, informative, clean and powerful.