I think you overpitched.
I use 1600mL on a 12-gallon batch, so using 2L on a 6-gallon seems to be a bit much.
I had a Belgian Golden Strong Ale that we made a few months ago. Made a starter and everything seemed okay. Pitched the starter and fermentation was sluggish. I had to repitch another starter to get it to ferment out. When it was done even gelatin couldn't get all of the yeast out of suspension. I haven't figured out what could have caused the problem, but more than likely it was low initial fermentation temperature for the yeast I was using (thanks, 20/20 hindsight).
I suggest making your starter at the temperature you're going to pitch it and make a smaller starter, something along the lines of 600mL - 1L tops. I usually pitch 18 hours after I make the starter without cold crashing or decanting.