So, I still haven't gotten around to taking proper measurements of these speakers - I will, eventually, because I'm dying to know if my ears are useless and I'm just not hearing that huge woofer dip that you've observed - but I did try out a simple mod by replacing the stock midrange with this cheapo
GRS G-SMB1-8 5 1/4" sealed back midrange that was conveniently the only appropriate driver Parts Express carries. Its sensitivity is 93dB so I figured it was worth a shot given the speaker's 95dB spec and the possibility that it could be slightly overstated as Amir's tests have shown is somewhat common for commercial designs.
It was ever so slightly large for the baffle cutout, but I "finessed" it into place with the mounting screws and ultimately it fit without much trouble.
Subjectively, it is a significant upgrade. I think the slightly lower sensitivity improves the tonal balance of the speaker, to the point where I was able to remove the +3dB bass shelf I was crudely adding with my Yamaha AVR's EQ curve. They still rattle my windows and the sound is now much smoother with none of the harshness in the lower treble that was my least favorite aspect of the speaker. I think for the price this midrange is a very solid performer, and for $15 it's a no-brainer upgrade that doesn't seem to require a corresponding crossover mod. It also reinforces my gut feeling that the tweeter is totally decent given the price and the possible pitfalls of the horn.
Even though the cabinets are unbraced, they don't seem terribly resonant to me. I'm currently playing around with modifying some 90s Sony monkey coffins that I rescued from my parents' dumpster (probably a fruitless endeavor) and the difference compared to the BICs is pretty striking. Nonetheless I will soon be gluing in some strips of MDF and adding batts of rockwool to see if I can take them just a bit further.