>>1. Could I build these as towers, with a taller footprint (probably around 30" high) and build
>>a divider in to tune the cabinet properly?
Sure - or build the speaker as normal, and build a matching stand that it fits on so it looks like one piece from a few feet away (for flexibility, in case you don't want as tall a speaker later on).
>>Would it make sense to fill said cavity with sand for damping?
You could do that, but personally I'd probably just brace the heck out of that "dummy" space, and fill with insulation, to avoid making the speaker a lot heavier.
>>bwaslo's build thread for these lists the DNA-360 driver, but Erich's parts page lists
>>the DNA-350 - any benefit to going with the more expensive driver?
Not sure if any crossover changes would be required, bwaslo would be a better source for that - but yes, the 360 is a bit better than the 350.
>>One other completely unrelated question - what is the purpose of a sand-filled stage? Is it just
>>to provide a solid, decoupled base for your speakers/subs, or does it have other functions
>> (outside of aesthetics). Sorry, I've always wondered this....
That's about it