I have a new sub so have been taking some measurements recently. I noticed something interesting that had never occurred to me before, not sure if this is stating the obvious to everyone else but thought I'd comment anyway
This is the raw response of my sub in my completely untreated room, I have grown to know and love that 40Hz spike
the 60Hz null goes if you move the mic a few cm so ignore that for now
I was just doing some quick compression testing so 1 PEQ filter & a few sweeps later produces
I stopped at this point as my door frame was creaking! distortion for the top sweep
Notice the substantial elevation in distortion around 20Hz in the 2nd harmonic and ~13.3Hz in the 3rd harmonic & note how it falls either side of that frequency. If the sub was being overdriven (I know it's not, the drivers are not even working that hard) then you'd normally see distortion continue to escalate as frequency drops. My thinking is this is an artefact of using EQ to tame a mode, i.e. the EQ targets the fundamental frequency only but the mode still exists and still serves to amplify the harmonic distortion from lower frequencies. Hence you see this "artificial" (but still real) boost in distortion. This sounds plausible to me, any thoughts welcome though.
This made me think about building a tuned bass trap to tackle the 40Hz mode instead, not sure whether it will be practical in my room but worth investigating anyway. Does anyone have any experience of building/using such a thing?