Personally, if I was gunna do that, I would prefer a traditional 3-way studio monitor layout with the subwoofer to the outside of the woofer/tweeter combo. (Like the ATC SCM25A)
It would still be massive and heavy. but hey, 20-20K with full control over the low end, even with the smaller HX310 amp? I'd be game for that.
You would still lose some of the imaging/soundstage benefits of a narrow baffle, but the waveguide will help as well.