I appreciate that you are mentioning the placement of the base of the neck. That is more important than whether the head is up or down. If the base of the neck is low by birth, it is harde for the hrose to raise it, and rock back onto his hind. You won't get that nice arch to the neck that Mr. Livingston shows in the extreme (I LOVE him!)
Also, though, look at the size, length of shoulder and angle this is fromed by the humerus as it joins the shoulder, and the angle of the humerus joining at the elbow.
That halter horse's neck is tied in pretty high, but his shoulder, (what I can see of the bone structure under all that overly ropy muscle) is actually rather small.
Not sure the angle, cant' see the photo right now.
But, I'm saying that the shoulder needs to be considered WITH the neck.