Well, I've never studied anyone else's style of Xingyi, and I didn't mean to imply that san ti doesn't tuck the chin, since I know that it does.
San ti, at least as illustrated by Sun Lu-Tang, has one hand forward and another hand in a low guard, exactly where you would end up after Pak Choi. Our guard stance has both hands up around the shoulders, with one slightly ahead of the other, and the body diagonal so that the lead hand and the lead foot match. It's just a less extended stance, and we give up some centerline protection (of the kind found in San Ti and the Wing Chun guard) for better head and rib protection. Sadly I have no illustration of this.
If I ever make it to Singapore, I would love to spend an afternoon training with you, and learning how your Xingyi differs from ours!