A follow up:
I've been trying to figure out what's going on with my setup. I'm just auditioning two receivers and though I've pretty much ruled out one of them, I'll be adding another in a couple days. I'm trying to figure out what problems are associated with the head units Vs speakers Vs resonances.
I just tried throwing thick towels in front of the two tweeters so as to narrow it down to my door coaxs. I'm definitely hearing honkiness with vocals (ie midrange) and it's really bothering me. I previously added deadening to the outer skin, but putting my hand on the inner panel adjacent the speakers, I can feel vibration, particularly during significant midrange content (together with some during deep bass.) So I'm thinking I need to add deadener to the back of the inner panel, at least in the region of the speakers.
I also noticed that when I remove the towels, and trim back the treble to maintain balance, there actually appears to be an increase in honkiness. Makes me wonder if the tweeter is contributing some in that frequency range.
MidlifeCrisis -- You say you changed the filter capacitor?