Well, at 6', you are right at the upper max height limit of the Expedition. So that might be a factor. People of your height and even a bit taller use the Expedition and are happy, but many others among us find the longer hammocks more comfortable. For example, I have noticed that Expeditions are for sale here at a factor of 10-1 compared to Explorers. Maybe that is because way more people buy Expeditions, or maybe the longer ones are more comfortable for more people, hard to say.
You are also fairly close to the weight limit on the Expedition, so that might be a factor keeping you from getting flat enough. Maybe too much stretch for optimum results? The BB 1.1 dbl might be just right for you with it's 275 lb limit. The 1.7 dbl has a 400 lb limit, but while it would probably be "flatter" for you with it's greater reserve, it might not be more comfortable for you for a variety of reasons having to do with stretch. Again, hard to say.
But more folks love BBs than anything else, so give it a try. Talk with Brandon about if you should get the 1.1 dbl or something else. If for some reason that still does not do it, don't give up. You can likely sell it here quickly for little loss. Then maybe try a HH DJ XL or Explorer. Or better yet a Safari, though it might not have some features you want, and weighs a few oz more than other hammocks even though it is not double layer(but oh so big and comfortable!). And if you want to try something dif at a much lower cost, don't forget the 10 ft. long Claytors. Though a narrow hammock, I have found these to be very comfy, my Claytor No Net remains one of my overall favorites. Also, some folks are speaking highly of these, though I have not yet tried one: http://stores.tttrailgear.com/StoreFront.bok
And lastly, if laying flat is the problem, there is always a bridge hammock:
It has it's own pros and cons like all the rest, but Laying FLAT is not a problem with this hammock!
If possible, get yourself to a hammock hang get together where you can try a bunch of different hammocks to see what is right FOR YOU! Happy hunting!