I assumed it might have been the installation depth issue with the window rolling down. With a deeper spacer ring so that the gtos would clear the window, was there not enough space between the door trim and the speaker itself? Or were the two touching?
Sherry also had some nice Infinity dampening sheets. Dirt cheap too for 1500 a sheet. Whether you want to do it ( now or in the future ) or not, it's a great way or really amping up the sound of the speakers and you eliminate a lot of that extra resonance