But 30hz is 'wasted' if you're using a sub... So basically, the towers would be designed to be run full range? Anyhow, I guess people always want more. I've not heard the Towers nor can really remember what the exact designs goals were for the Sierras, but I doubt they were max SPL and max bass extension... I have the Sierra-1 and the bass is very good for bookshelves. So I'm guessing twin woofers optimized for bass, would be quite decent... Is it the same as dual woofers of the same quality but 8/10/12 inches? No, but I guess it depends what you're trying to accomplish.
So the larger towers would be made to be run fullrange? If so, my question is, would a pair of Sierra Towers + sub(s) would be better or worst? The sub would dig deeper, would be placed in a position superior for bass than where your mains are, and the mains would just have to handle >80hz which they are probably very able to do...
I guess larger woofers
might improve mid bass, but not having even heard the Sierra towers nor having any substantial speaker engineering knowledge, I'd just answer maybe and wonder how much more it would cost for the drivers and cabinets... Another question is how 'doable' larger woofers are, I think I remember something about the efficiency of the woofers matching the mid or something, anyhow, above my head...