I was using a adjustable back system rucksack,with it definately sat on my hips, belt nice and snug and i still got cramped trap's,granted i had the shoulder straps pulled tight,but any loosening of them caused the pack to drop further down and that felt very awkward and unnatural on my back, like i said earlier i think because i had'nt done much distance with a heavy-ish pack for a while that added to problem, but even in my younger fitter days long distance stuff always got my traps more than anywhere else,could possibly be a genetic/body shape type thing?
I'm used to having the pack as high up as possible, probably because I wore one with webbing. Your trap's will get used to it though.
Although I like the look of the old school knapsack shape, unless they had an external frame I've never really got on with them as I found these pulled downwards.