Rip,
I've been videoing my squats for a while and I have some friends who have done SS much longer than me comment on them. They all agree that my knees are traveling too far forward and that my weight needs to be more on my heels.
I've tried both toes-up and the TUBOW but if I set it closer to my toes I either knock it over or fall on my ass. Haven't tried box-squatting yet.
I'm tempted to think the cause of all this is my long limbs. I'm also battling a couple of spinal disc herniations and have pretty tight hammies.
http://www.youtube.com/watch?v=3vkoTFjOXe8
Thanks,
Johann