10dB resonant peak at 40Hz

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.

At that frequency, a high pass filter at 50 hz might work.

I had a similar issue (12 dB @30 Hz), and was able to work with Music City Accoustics to get some wall-mounted tuned bass traps. I tried to work with GIK on this issue, but I found them to be inflexible and uninterested in addressing this targeted issue, by the way.

I am not sure how constrained you are when you say"minimally intrusive." These take up no living space but they are visually apparent. Corner mounting was not required. I ended up buying 4 of them. They are about 14" square each, and they are mounted in a pattern on the wall. I consider them unintrusive, and my SO had no complaints. You/your SO may disagree.

The effect of adding these was immediate and extremely effective. I wouldn't want to address the problem any other way.

I have a couple of GIK Acoustics corner bass traps that are not very intrusive and I'm not using them.  If you're near SD you're welcome to try them. 

Otherwise, look for some near you.  An easy solution if it works.  

@OP - Can you move the speakers closer to the front wall? Even if you still get a resonance you just don't want it on a root note that bass players land on regularly - but even a relatively small movement might tame it anyway.