I had a pair of those for a while and had the same thoughts regarding upgrading the woofer section. My condensed thoughts on the speakers as a whole:
The designer/builder was very helpful in answering questions - even on my second hand speaker purchase.
There is some big potential in upgrading the stock crossovers in these (Sean listening maybe?).
These are very power hungry. If you've only heard them with a lower powered amp, a big amp really makes them a whole new speaker in my opinion - and much to the better for it.
I don't remember/think they would fit on the woofer cabinets as built, but the front baffle width was just about big enough to get a 12" driver in there, if the box was rebuilt to accommodate the volume if one were so inclined. I'm not sure a single 8" would be enough to get enough drive in that range to completely solve the output problem; quality would improve, but if you're looking for displacement, I think a sealed 12" if you were going that way would maybe be better.
The upper baffles are easily able to separate from the woofer cabinet, for modding purposes. Also (at least the ones I had) were set up for biamping split between the woofer and the mid/tweeter section, so just running the uppers was also easy to experiment with (not sure of the impedance with just that section playing, though?).
My one real gripe was that the vertical off axis response of the tweeter was really not so great. The tweeter panel is only about half the height of the mid panel. If you listen seated at tweeter height, things are great; If you stand up, the top end falls off fast. In my "perfect world", there would be a version of this speaker with a full-height tweeter.
Regardless of nits to pick, I have to agree that with these you get a lot of speaker for not a lot of money.