Definitely don't bother with the Pro-tec ones, they offer VERY LITTLE tailbone and butt protection while focusing their padding on the hips... which i have no idea why. The Burton RED shorts are terrible too with minimal padding. Their original spandex type ones seem pretty decent.
Aside from the original RED spandex ones i'd look into other sport ones. Definitely try to see them in person before buying to see where and how much padding and protection they actually provide. While i don't wear any impact shorts, there were definitely a few times this season where i wish i did.
Azzpads work well, but it is like wearing a diaper, its huge I got one that's like 2 sizes too big for me and never used it out of embarrassment, though it was good for mountain boarding when no one was around to see this huge black diaper thing on my butt.
Any decent protection is going to have some bulk to it. I think Skeletools are the best out there though. Their shorts are very effective and yet are some of the least bulky out there. Of all products I think they are the best.
I ordered the sixsixone's after reading this thread. They seem to fit well under my pants and are fairly comfortable. I haven't rode them yet, just had them on in the house. THey are thin. I'm doubtful they are really going to provide much protection given the minimal thin foam padding. I was hoping there was more protection around the tailbone, but just one thin pad there.