MikeS, first I want to thank you from the bottom of my heart for the lead to deepsquater.com. Not only was the box squat article incredibly illuminating, but there are tons of other great articles in the archives section as well. Plus, I suspect its a great Forum. For those of us who don't have a strength coach, it is so hard to know if you really are on form or not, because so many others in the gym could be more wrong than you are. It is so hard to find a truly accurate reference point, and clearly deepsquater provides great insight. It is worth remembering it is geared to powerlifting, not bodybuilding, but in terms of form and basic lifts, it seems wonderful.
Josh, yes, after reading the article and thinking about your comment, my concern about sitting up straight (perpendicular) made no sense, as you said, you begin standing more or less straight, and don't fall over then. The arch in your back creates a platform for the bar to rest on, even if you are "perpendicular". What i found most interesting and educational is the concept of going "back" not "down" in the squat, which can be exaggerated with the box, and I am confident that after a bunch of box squats my form will improve considerably.
NOW, the MOST interesting thing is every deep squat article makes clear the muscles used in a good squat are the dorsal ones--back, hams, glutes along with abs. NO mention of QUADS. While of course they play a role, it doesn't sound like the squat is really meant as a Quad exercise. That being the case, what IS the best overall QUAD lift??? maybe leg press or hack squat? Lunges? It sounds like the quads are ancillary muscles for the squat, not the primary drivers, and as such, will not be maximally stressed. Interesting perspective, since we have previously decided the squat is the best quad lift there is. This is not meant to be critical of all of us, it was a kind of "ah ha" experience for me to think in those terms, but it opens the door to the question how best to work the Quads, which is where I basically began the thread.
By the way guys, thanks for having patience with me, I really am trying to think this one through and get to the bottom of the most effective way to work quads, and I feel I've learned a lot about squats, but still am not sure on the quads.