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.