I have a BB and love it to death but I hear what "jerseydrew" is saying. I keep my settng's on the lite side, but I love the end result! Another one you might might want to check out is the AC Booster. They are all great so it comes down to music style and taste. "richb" has a Mark II C+ and also uses a BB Preamp you might might want to shoot him a PM.
I was over his place a few months ago and he is the one who introduced me to the BB Preamp at the time he was using a Mark IV like I currently do I ran out that weekend and got one for myself. But Rich is like me (or I should say I'm like him, he had the BB first) keeps the setting on the lite side.