10dB resonant peak at 40Hz

My room is about the same dimensions and my last sub. , a REL was used with a Velodyne SMS 1 .   It pulled down that nasty peak.   The sub that replaced it has built in DSP and it also does a good job of getting rid of the boom

Before you do anything, consider an Auralex SubDude isolation platform.  Cheap and effective at taming resonance. Start there then explore DSP solutions.  

Yes, DSP would do it. I'm not using a sub.

Last speaker was the JBL L82 and even the KEF LS50 Meta (which I did not think even went to 40Hz) does it. If I plug the speaker ports, the resonance goes away but so does too much of the bass.

Well, if you could move the speakers I’d say use the AM Acoustics room mode simulator to try to put your speakers and listening room out of the 40 Hz mode. Otherwise EQ is your only option.  If it makes you feel any better, it's a very good option as well.

@erik_squires you mean THIS?

I’ll try moving them around this weekend. Right now, they are 50" out from front wall and 30" from side walls. I did calculate 15 ft resonance is 37 Hz - sounds about right.

Since plugging the ports worked, sealed box may be another idea but would need a larger cabinet.

OP:

Yes, that’s the one.  As you mention, plugging the ports will work but sometimes at the expense of all lower bass.  A parametric EQ on the other hand can clip these enormous resonances effectively and make it possible to better balance the bass output.  With subs clipping peaks can often help you raise the sub volume very effectively.