Dave,
I have used a 150 grain sink tip line with this rod and have found that it cast fine. It is an Orvis line and I think that the sink tip portion is 24 ft long. You will want to cast it like a shooting head. That is, strip the line in until you have only the sink tip portion out of the rod tip. Don't try to pick up the line if you have much more line out than that. When you cast, don't allow too much "overhang" of the sink tip portion beyond the rod tip. I find that if I get more than 10 feet or so of the running line out it doesn't cast well. This amount of line should load the rod well so that you can shoot whatever remaining line you need to reach your target.
Have fun with it. I really like my Biix 6wt, so long as I don't try to overload it too much.

Ted