Total bass suck out at 40hz

A 40 Hz null is typical of a room node, as noted. Here’s what worked in my room, where I use two subs and had a 43 Hz null. I moved the subs, measuring at each position. Putting furniture glides ("supersliders" under the feet helps with moving.) The position where the bass null was minimal put the subs behind the listener, near the rear corners.

I agree with others that a distributed bass array of at least 4 subs is a great solution, but I didn’t have room for that.

I also agree that asymmetrical positioning of the subs can help, but I wanted to keep them symmetrical and running in stereo.

The Allison effect (floor bounce cancellation) was mentioned; my impression is, it is typically higher in frequency (100-300 Hz).

SBIR suckout at 40 Hz might be a factor with speakers 7 ft from a boundary, more so for speakers 7 ft from several boundaries.

Oh, one other thing . . . do the subs have continuously variable phase controls, and have you adjusted them for smoothest FR? That can be an important step in many cases. As can be changing the xover frequency, especially if running the mains full range.

I hope that helps.

They do have continuously variable phase controls. I;m not sure what to try there, I guess trial and error. I have spent the last hour or so just moving them around randomly and re-testing. I have gotten audible base in the 40hz range but it is still down 13db from the 30hz tone. So do I have a peak or a null? I have 90db at 30hz and 89 at 20. The 30 hz tone is clearly audible the 20 not so much but I can feel it well. Back down to play with the phase control and see what that does. I'll be back up later to see if there are any more suggestions. And thanks to everyone who has tried to help, I really appreciate it, Allen. (AKA Mizike)

Regarding phase, there are a several ways to adjust. Here are the two I think are best. In each one, you run ONE speaker and its associated sub at a time:

1. Put a mic at the main listening position (MLP), run a sweep, and analyze with something like REW. Move the phase control to get the smoothest frequency response (FR) around the xover frequency. This is the most direct and reliable way, in my opinion.

2. Sit at MLP, having reversed speaker leads of the main to put it out of phase. Play a tone at the exact xover frequency. Adjust phase control to give the best null at that frequency. (Remember to put the speaker back into phase when done!) This is the best way when you don't have the measurement equipment for method 1.

Any phase adjustment depends on positions of main speaker, sub, and listener. If you move anything significantly, it will change, so needs to be redone.

Hope that helps.

I have just went through the entire range of phase adjustment, moving the nob three clicks at a time and playing test tones up to 60hz. Above 60 nothing really changes no matter what I do.I have made the mode better but it is still there, down 8/9db from 30hs and 5/6db from 50hz. (i have some loss of volume at 50hz as well, although not nearly as much) I do notice a definite gain if I move my head and/or meter forward and down towards the floor. Maybe I should just get a much shorter chair. Or consider building a platform for one or both subs. 

Sub crawl. Put one sub exactly at your listening position. Play the 40 hz tone on continuous repeat. Get down on your hands and knees, and carefully cover the whole of the listening area--some kind of grid pattern is good so you know where you’ve been. Mark any spots where you can actually hear the tone. Hopefully there will at least two. Move the two subs to the marked positions, and return to your listening position to check result.