 This method should remove all doubt as to whether the preamp circuit board is a IIC or a IIC+.The Mark IIC+ is currently the most coveted vintage Boogie, selling for twice its original price on average, because of its much praised "Liquid Lead" mode, and also its warm, clean rhythm mode.12 Factory C+s with switchable ++ are known to exist, before this idea was used in the Mark III, with the ++ being developed further to become the R2 channel on the III.