I haven't made a wee heavy, but I did do a scottish ale that I used the 'boil down a gallon to syrup' method on. I did the recipe twice in quick succession. The first time was with the boil down and the second was with crystal malts. Of the two, I preferred the second. Granted, I'm sure this has a lot to do with your burner, boil intensity, and how far down you let it go yadda yadda.