Sorry for the delay, took some time to get around to properly sit down and do this. I can report back that this indeed did the trick, though I didn't have to trim off a whopping 15 dB for 131 Hz, as 6 were already enough to get rid of the droning. For 75 Hz I don't really feel much difference in terms of droning, but I gave it a reduction of 1 dB for good measure anyway. The Q's were left unchanged from your suggestions though. With these settings, the droning disappeared almost entirely:
Now, however, I do have to admit something. I did experiment a little more with speaker setup and found out that, despite my initial assumptions, indeed it wasn't the subwoofer that caused this, but the monitors after all. If I turned them off and only let the sub do its thing, it was clean as can be, no droning whatsoever. I feel very stupid now and I could've bet a tenner on it being the felon. But here I stand corrected.
Whatever the case may be, I'm glad it's fixed now and I don't have to spend ridiculous amounts of money for these absorber thingies. They'd probably take care of my problems, but I neither lack the space nor the funds to put them up, so that's gonna have to wait for a while.
Thanks for your tremendous support!