I'm confused by your description of the size. You say massive, but then you say that their backs were level with the guy's knees, which is a small dog.
Here are a couple of pictures that I just took a few minutes ago, of my 150ish lb dog with my 11 year old average-sized son. His back is about 2 inches lower than my hips, and I am 5'6". People have made comments when passing us, indicating that they didn't think that he was a dog at first

:
http://picasaweb.google.com/paperdov...bpoeuDib6EmQE#
Your description of the dogs pretty much fits my dog, with the exception of coloration (and a pig nose?). He has a definite neck, but he has a HUGE mane of hair so it looks like he doesn't have much of one. And he generally holds his head lower than his shoulders (it is heavy!).
Also, a dog with a double coat, with black guard hairs and grey undercoat, can appear as you describe- sparse black hair with grey skin visible. It's all hair that you're seeing though.
Anyway, going with the assumption that these were large dogs, could it have been this:
http://www.moloss.com/001/breed/def/d007/
Or this:
http://www.koinfo.org/English.htm