REL six pack questions

Hoping those who have experience with REL six packs can answer a few questions for me. I am building a six pack and will have it operational in the next week. I am having a local dealer set them up, but I bought them all used and would like to ask the forum a few things. Some of these are just curiosity, but others I really need to know.

1. I watched John Hunter's six-pack setup video and cannot  understand why each sub is set up progressively without the other subs stacked above it. One would think the physical presence of the other subs above would affect the sound of the one below it. Why not have all three subs stacked up, and plug one in at a time to adjust it?

2. Speaker designers spend painstaking years getting a speaker to be very close to flat in frequency response. With a six-pack we are deliberately dialing the upper sub crossover points to overlap with the mains (my mains are flattish down to 28hz). How does this not create a muddy mess?

3.  My understanding is that part of the real magic of six packs (spatial realism, textures) happens because there is hidden low frequency content in fundamental instrument/vocal performances that becomes apparent with a six-pack. But if one has giant mains like Magico Q7s, for example, they produce deep frequencies. Why would a six-pack help mains that also produce very low bass? 

4. Separate from the above magic, I have also know that low frequency musical signals create harmonics all the way up the spectrum. But the subs don't reproduce anything above the midbass region due to the second order crossover so how would the subs reproduce those higher harmonics? Or maybe the point is that the six-pack reproduces low bass one didn't hear before, so now oe hears new higher harmonics that were not there before?

I have another question but I am going to post it separately because it is a special topic. Thanks for any thoughts on above. I am fascinated and excited about the six-pack.