Good day:
You mentioned insulation-too much can hinder bass extension in my experience-typical thing is to line one opposing side of each pair of sides from what I've gleaned on this site over the years.
I didn't read if you were using new GPA drivers or 604-8K's, when I glanced back briefly. New drivers may have very stiff suspension, as I know from a 604C re-cone from GPA versus the springiness of the original cone. This may or may not affect LF response, and if so does it need time and use to improve? I don't know and have not been able to tell from a forum search. I have not used mine long enough to tell.
What about trying original Altec 604-8K crossovers, without the tweaks? I have -8K's in Stonehenge-style aftermarket enclosures, about 8 cu. ft., with orig. Xovers and they sound great- strong bass is there when the program calls for it, but no bass hints or boom ("undertones"?) when there is no bass info.
-Jon Archibald