going from 2 subs > 4 subs

Front wall corner positioning usually leaves room modes unchallenged. Four subs will reduce, if not eliminate the rooms modes altogether as well as the timing issues. Requiring less gain from each sub and creating a wonderfully tight room lock.

Personally, I’d map the rooms bass modes by using one sub at the listening position playing test tones and preforming the crawl test. Relocating your current subs near those modes and daisy chaining them wirelessly or using inexpensive custom length RCA interconnects from Blue Jeans cable.

Properly positioned, two subs should load the room more efficiently and reduce bass modes. Adding DSPeaker or a third DSP equipped sub with greater low frequency response would become the master to slave your current subs to produce the character your looking for from your subwoofers. They need not match.

In theory additional subs should be more beneficial in small rooms because of long (low freq) wavelengths. Of course the biggest difference is in "flatness" of response in multiple listening locations.

What I experienced when going from 2 subs to 4 was the ability to better fill measured dips in the frequency response. However, that was not particularly audible. What was audible (as pointed out by @fuzztone) was the smoother bass response across space. Now, when someone else is in the sweet spot and I'm at the side, I hear sound that is much closer to what they are hearing.

Thank you all for taking the time to respond and to offer such helpful advice. Although I understand a bit about frequencies and room modes and non/omnidirectional subs, I couldn't help but wonder if having four subs, including two *behind* me, would change the presentation of the full range of music, perhaps making it more "immersive" or opening the soundstage. Everything that you're saying about a smoother bass response does make complete sense. I'm just going to have to try it and see (or hear)!

The only way subs can affect soundstage is ala Jim Smith's Get Better Sound.

I recommend getting a $100 Umik and REW software and measuring.

I get great sound with 2 tower speakers and 2 REL subs...with great help from Duke (the designer of the Swarm) on set up...the keys were looking at my large towers as subs (maybe different if I had monitors) and subs placed asymmetrically...initial placement was easy with Duke's guidance, then a few weeks of fine tuning...