I've read this interesting question, and it made me wonder. The fact of tilting/raising the speakers is rather obvious to me, however there's one possibility I thought of.
Typical big-stage setup involves either one (angled or flat) 4x12" cab and a guitar head, or two cabs, one on top of the other (then typically the lower one is flat, and the amp resides in the backstage). However, many 2x12" combo amps offer an option to work with their built-in speakers and a separate cab simultaneously.
Would it be safe to put such a (which would weigh about 30kg, compared to the head weighing around 15) on top of a cab? The problems I was able to imagine are
- A combo is vibrating mechanically way more than a stationary head. Could the vibrations move it so it could potentially fall?
- The combo is heavier, so could it damage the cab itself? Could the added mass dampen or affect the sound of the cab in any way?
- Because of that, the assembly, i.e. putting the amp on top, would be way harder. I've also seen some cabs having holes on the top to hold the above cab in place; I doubt the combo's legs would fit them.
- The angled cab has less space on the top, so I guess only the flat ones (that also should be a bit more durable because of that constructions would be elligible there, I guess.
I think it could be an interesting and rather affordable setup, but there might be other things I'm overlooking.