Think about this... when a recording engineer is mixing the music for recording... he's doing all kinds of processing to get it to sound like he wants it to sound... and very likely in a room well treated with acoustic panels. They may use headphones yes, but I've seen high-end recording studios with speakers to listen to what they have recorded. They treat their studios to listen, why wouldn't we treat our room. They use processing including equalization to get it to sound like they want it to sound... why wouldn't we do the same.
I've heard some pretty good setups with nothing but panels... but never heard any that did not sound better with good EQ processing if it was available. Not saying they aren't out there... I've just never heard them, and I have listened to a bus load of rooms with both, especially over the last 10-12 years.
Also think about when that recording engineer works on the sound stage and imaging of the recording. He is counting on frequency matched speakers to get the sound stage and imaging correct. Now consider the average room and someone setting up a pair of speakers in that room... likely a great room, living room, den, etc. There are all kinds of furniture, fixtures, doors, door openings, windows, curtains, blinds, hard surfaces, soft surfaces, different heights... any combination of these, which means those speakers are hardly going to be frequency matched at the listening position, which also means it's going to be hard to hear the imaging and sound stage that the recording engineer heard in his studio. Wayne could identify a mere 3-4 db mismatch of certain frequencies in speakers in my room... because he knew the imaging of a certain song was not correct, or what he thought it should be, or as good as it had been in previous listening. Once it was corrected... all was good. Unfortunately, it is doubtful that acoustic panels alone will correct the frequency mismatch of those speakers in that great room... I just don't see how it can, although I am certainly not the expert on panels... and I have panels, mainly to help with reflections that shouldn't be there... and to help with the look of the room as well (subliminal perception of improvement). Actually Wayne identified some first reflections off of my leather recliner, which now has very think plush covers over the top that solve the reflection... I didn't notice it, but he has ears to die for. My thinking is that you pretty much need to frequency match those speakers in that great room as best you can with some sort of good equalization... and I can't see where limiting that EQ to 500Hz is going to solve all the issues likely present. It needs to be full range. If you have issues at 1kHz or 2kHz where the speakers may be out of match by 5-10db... it has the potential to cause issues with sound stage and imaging. This is why in nearly every case where we've done evaluations... using equalization improves sound stage and imaging. With some speakers it is more drastic than others because of poor consistency of speaker manufacturing... and/or defects. Imagine the cone of a tweeter being pushed in and interfering with imaging... and then seeing it on the measurements (one of those speakers with multiple tweeters). It took us a while to notice it, but finally we started measuring each tweeter and noticed one was pushed in a little... pulled it out and all was good.
Panels will not address nasty peaks below 80Hz, unless you have some computer generated gizmo that can analyze your room and figure out how to build an elaborate system to trap such frequencies... and that surely ain't gonna be cheap. I've investigated it fairly extensively... and quickly decided I couldn't afford it. That 10db peak at 30Hz is going to require something serious... it makes sense to me that the easiest way to tame it is with EQ. This is one reason I've never been a proponent of those systems that do not dedicate very many... or in some cases zero filters of resolution for the sub bass. Those nasty peaks can effect your imaging drastically and create one note bass.
I am not saying you can't get good sound from only using panels... but I do believe it can be made better with proper EQ. I prefer both... and I am a HUGE fan of Dirac Live V2... can't imagine living without it... just take it all if you take DL from me.
