I was thinking about starting a thread about this very topic. A couple of days ago, I was listening happily to my system when I noticed a vocal resonance that I didn't like coming from my fiberglass kickpanel. I knew it wasn't the speaker, but the actual kickpanel itself vibrating at what sounded like 150-250Hz range. I recent put my highpass on the kickpanel to 150Hz.
After hearing that vocal resonate the kickpanel on that track, then I started hearing the same resonating on acoustic guitars! ARGH! I HAD ENOUGH!
So, yesterday after work, I went to town on my car trying to figure out how to minimize or remove the resonating tone on vocals and some acoustics bass instruments.
I turned up the highpass on my amp to 250Hz(it's max). I turned off my subs and disabled my midbasses. Then played a few seconds of a track that resonated the panel over and over and over again. Making EQ adjustments and volume adjustments then realizing the slope of the highpass just wasn't steep enough at 18db/octave to kill the upper bass resonance.
So, I re-connected the active crossover's "high-pass" section to the RCA line going to my amp's kickpanel input jacks. (I was just using the active crossover to lowpass the midbass to complete a bandpass.) So the active crossover's highpass was turned up to 300Hz and used in conjunction with my amp's 250Hz highpass to effectively give as steep a slope as I could get.
I found I actually needed every bit of that slope and maybe a tad more. Finally, at moderate volume the resonance was gone and only at high volume 95db+ SPL does the resonance get audible to a small degree. I lowpassed my midbasses at 270is and boosted their output a tad then compensated with a lowering of 160Hz & 400Hz (400Hz for the lack of lowpass filter steepness to match the highpass on the kick) on my EQ while keeping the deeper bass at 60Hz up at normal output. I found my midbasses play this upper bass region without resonance because the door is so well treated.
I reblended the sub-50Hz subwoofers in and then after a bit of midrange EQ trial and error ended up with a beautiful sound with no resonance and just this velvety sound. Add to that, being able to hear deeper into just about every mix.
After achieving this tuning result, I couldn't stop listeining to music. People who passed by must have thought I was crazy. I sat in my car for 3 hours enjoying track after random track and hearing things I either didn't hear before or haven't heard in years since I had a good home audio system. A particularly pesky track that makes my computer speakers resonate, and my kicks resonate is the Brian Eno - Dune Prophecy track. There is a very high synthetic upper-bass tone that hangs that makes most plastics hum and resonate that are directly coupled to the driver. I think the ensolite under the fiberglass perch in the door has decoupled the midbass enough to really reduce most direct resonances.
So, IMO if you can identify something in your sound or acoustics that you hear that just craps on the experience, tuning or adjusting can fix those issues. I think I'm just lucky that my midbasses in my doors are acoustically treated well enough to be able to move a problem frequency to a location where it doesn't give issues.