10dB resonant peak at 40Hz

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.

Another thought not suggested above -- add a high pass filter to the existing speakers and add a powered sub or two. You can try the sub in different positions to find the best spot to even out the 40 Hz resonance.  Subs can often be easier to move into different spots without bothering the other uses of the room as much.

As an additional observation, I've noticed that sometimes the placement of the main speakers which gives the best imaging and soundstage may not work as well for bass resonances, and vice versa.  Since most of us don't have dedicated sound rooms, this means we have to co-exist with home decor and such.  Using a sub can be a good way to handle things.