All horses are built differently and will carry their heads comfortably when they're allowed to. I believe the key to getting a horse to "carry its head low" (rather, collect itself) starts not in the shoulder or neck or mouth, but in the rump. A horse that is driving from behind will round itself out and most often will not get above the bit, which gives that horrible hollow-backed-nose-out effect. Anyway, back to what I was saying about horses being built differently... consider western pleasure and the wide variety of horse breeds that compete in it.
Arabian in WP:
QH in WP:
The same discipline, two completely different looks. Never resort to pulling on your reins and strapping all sorts of devices to your horse to get his head "down".