The Mark IIC featured a quieter footswitching system and a new mod to the reverb circuit. According to Sacks, The reverb was noise-ridden on the Mark II, a problem which persisted with some IIB models as well. The solution Mesa came up with involved resistor swaps and a change in ground lead placement. That mod[ification] is still on the books of official mods, which they send to their authorized techs; it runs about $50.] Mesa/Boogie no longer does this modification at its own factory.
The Mark IIC+ was the last of the Mark II series and featured a more sensitive lead channel and, more importantly, an improved circuitry in the effects loop. Unlike earlier Mark II models, pedals could be used without the amps signal overloading their inputs. However, the volume pedal option on the Mark IIB cannot be implemented on Mark IIC+s. Some owners/dealers/sellers say the + refers to an amp having an EQ, but they are mistaken. The mistake may have originated in the mid 1980s, when Mesa/Boogie issued their Studio .22 model and then changed the name to Studio .22+, which featured improved wiring, etc. All the Mark II models could be made with EQ as an option, but not all of them did. A Mark IIC+ could, for example, refer to a 100 watt amp without EQ or reverb.
One can tell if a particular amp is a + by looking for a hand-written black + mark directly above where the power cord attaches to the back of the amp. Many dealers increase the price on a Mark IIC+ but often dont know anything about what the + means–they often dont even know where to find the + mark. Indeed, the mark itself can be forged. An owner can call Mesa/Boogie and ask them to check his or her serial number against their records. Mesa/Boogie only made about 1,400 Mark II amps before going to the Mark IIC+.
The Mark IIC and IIC+ are gaining reputations in vintage circles as the best Boogies, next to the classic Mark I, due to their much-praised rhythm channels, and to a lesser degree their lead channels.