Your post makes me very excited about how my new Pi 4's will sound.
How thick is your veneer ? Maybe the extra wall thickness removed or moved up some cabinet resonances. All I seem to find in Ct for veneer is veneered plywood. Where did you get your maple ? Or maybe I should use the plywood for extra wall strength, but my cabinets are already pretty heavy. Boy, I love the Pi 4's size though and the imposing look of the big 15" woofer... if only I had a bigger house !Did you use the Wayne's suggested bracing ? I used a different scheme. I put 2 by 4's horizontally across the top and bottom of the woofer, added 2 by 3's across the back panel parallel with the 2 by 4's, then additional 2 by 3's on the side panels. So I have a box above and below the woofer that supports all 4 vertical panels. Sounds confusing, but it's really straight forward. For the top and bottom, I just added a 2 by 3 across the shortest dimension, front to back. Seems like it should be tighter... sure was/is more work.