One suggestion - take a grinder and make a flat spot on the spigot of the D600. This is a common adjustment and useful for giving the strobe head more grip to prevent rotation.
So lets say the load rating for the d600 is 15.4 lbs. Does that include a counter weight, or not? And if it does, does that mean I have to find my own 8.5 lb counter weight for a 6.5 lb light? Because I can only find 15 lb bags.
I'm not sure about the true meaning of the load rating, but as to the weight distribution, that's going to be a matter of simple lever physics. With the stand point being the fulcrum, in most cases you see the light on the long arm of the lever and the weight on the short arm. That means you need a much heavier counterweight than the load.
If the load (the light and modifier) is a total of 7 pounds and you've got it extended on a boom six feet from the stand with the weight on the other end extended three feet from the stand, then you need a 14 pound counterweight.
Thanks for the heads up, RDKirk. Didn't even think about that one, but I certainly understand what your saying - a heavy bag will make sense to get then.
Probably will go with the D600, sounds very solid.