>>looking for the basic reasons why modern tower speakers seem to me to be taller and thinner (and heavier)
A couple of things :
Back in the "paper cone and rubber surround" days, you needed a big driver to handle the low notes because only a big driver had the cone mass required for a low natural resonant frequency, ie the ability to plumb the low notes.
Now woofer cones are made from all kinds of exotic materials (heck, someone's probably gonna make them out of metal one day
) so the mass (and resonant frequency) is no longer connected to the driver size. Smaller drivers can still handle the deep bass as long as you have the surface area and the cabinet size... ie the only thing different is that 4 small woofers are now pretty much equivalent to one large woofer in terms of deep bass ability, which did not used to be the case.
So... you can now have little speakers but you still need big cabinets, right ? Why tall & skinny instead of short and boxy ? First, obviously, is that smaller drivers make this possible. A vertical array of drivers gives the best imaging, so there's a lot of empty space on both sides of the driver column with a traditional cabinet now. A tall skinny cabinet is going to be stiffer than a cube-ish cabinet because you have smaller unsupported area. Also, I don't know the details but diffraction effects off the edge of the cabinet also help a skinny cabinet to provide crisper imaging than a boxy cabinet.
So... you still need "big", you don't need "wide" any more, and "skinny" sounds better than "wide". If you take away the "wide" but still need the "big" you only have two choices -- taller and deeper.
