with 5:1 it adds the third dimension... side to side soundstage and front to back phase too
5:1 is as great of an improvement over stereo as stereo was over mono. With mono, sound was one dimensional... coming from a single point. With stereo, you get two dimensions... a straight line from speaker to speaker. with 5:1 it adds the third dimension... side to side soundstage and front to back phase too. Ideally, an 9:1 system with high and low mains and rears would give you up and down phase as well. That would be true 3D sound. I'm sure that will eventually happen.
I have a quick nit to pick:
With mono, sound was one dimensional... coming from a single point
A point is zero dimensional; thus, think of the sound as a scalar
With stereo, you get two dimensions... a straight line from speaker to speaker
A line is a one dimensional object. The origin of the sound can be controled along a single degree of freedom.
with 5:1 it adds the third dimension... side to side soundstage and front to back phase too
The 5.1 configuration allows sound to be located in the plane, i.e. a two dimensional effect. Front/back and left/right are two degrees of freedom.
Like you said, speakers above and below would present a "3d" sound image.
I think it would be pretty neat to have a nice 5/7.1 system, although i could scarcely afford it. I just added the "1" to my powered monitors for 2.1 and now i need a good crossover which puts me far over budget. I can think of some other things that might help the sound more than 3 or 5 more speakers and would certainly be much more cost effective. And take up less desk space than a big receiver/ DSP!
anyway, there's more to sound reproduction than just spatial cues. I think headphones offer an advantage in performance/ cost for reproducing details vs speakers since they dont have to be high powered and have good crossovers and multidriver design. Niavely, i expect new headphones to outperform speakers from the detail/frequency response/distortion perspective vs new speakers at the same price point; however , things in this hobby are often over priced so it's not a hard and fast rule. Price does not indicate the sound quality of an item (cables comes to mind) so just because something is expensive doesnt mean it will sound good. Honestly, the whole question of comparing two anonymous items only by their price on paper sums up everything wrong with the mentality on this forum.
Cheers
PS running the subwoofer while listening to my headphones with crossfeed is pretty satisfying.
My paradox headphones isolate pretty well, but have good low frequency extension and the sub adds the vibrations. It's still only 2.1, but it's very immersive.