A compilation of Squat form critiques taken from Mark Rippeoe's Q&A
I really hate to tell you this, especially because you have such a cool garage, but not only do I find not much wrong with your low-back position in the first video 325 x 3, but it looks the same to me in the second video. And both sets were very high, the second set of five being about 3 inches above parallel. Your form looks okay, but I think the way to fix this is to shove your knees out on the way down, and make damn sure that they get shoved out at the bottom. Bigger guys have depth problems when the thighs bind against the gut, and you can test this by exaggeration: take a close stance toes-forward and squat down, and compare what happens with a normal width toes-out knees-out squat. When you get your knees out of the way, depth becomes much easier, and the adductor stretch this provides also makes the bottom rebound waaaay more effective. The addition of a weighted bar will provide the help you need to get low.
The squat shows a tendency to pull the chest up and consequently the hips forward a little past halfway up. This pulls the knee angle more acute and takes hamstring tension away, blunting the hell out of your hip extension at that point. This is often related to your chin-up position, and is one of the reasons I teach a downward gaze/ normal anatomical neck position. Driving the hips up without raising the chest is the way to preserve the back angle for as long as possible, thus allowing the hamstrings to work more effectively as hip extensors.