Cliff could you please brief us a little bit about this beauty ??
(From Wikipedia)
Mark IIC & IIC+
The Mark IIC finally remedied the two major problems of the IIA and the IIB, which were the noisy reverb circuit, and a footswitching system that produced a popping noise when activated. The Mark IIC featured a quieter footswitching system based on optocouplers to reroute the signal, and a new mod to the reverb circuit. The reverb modification 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."[citation needed] Mesa/Boogie no longer does this modification at its own factory. The Mark IIC also featured a new Pull Bass shift on the front panel, which slightly extended the low frequencies in the preamp.
The Mark IIC+ was the last of the Mark II series and featured a more sensitive lead channel, because it features a dual cascading drive stage, whereas the IIA and IIB had a single stage drive circuit. The IIC+ also had an improved effects loop. Unlike earlier Mark II models, pedals configured for instrument-level input signal could be used without the amp's 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 don't know anything about what the "+" means--they often don't 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 IIC amps before going to the Mark IIC+. Another cosmetic way of distinguishing a IIC from a IIC+ is the front panel. A IIC has the traditional "Gain Boost" pull switch integrated into the master volume, while a IIC+ replaced the switch with a Pull Deep bass booster. Some Coliseum series IIC+'s retained the surplus "Gain Boost" faceplates, though. IIC+'s upgraded from earlier IIC's also retain their original Gain Boost faceplates. However there are some very early "transitional" C+´s (in the 133xx range) with gain boost on the panel, but it works the same as the pull Deep on the later versions.
A practical, non-cosmetic method for determining whether a Mark IIC is a C+ is the "Loop Test":[4]
1) Plug your guitar into the Effects Return jack 2) Switch to lead mode 3) Turn the Lead Drive and Gain controls with a note ringing. 4) If they have NO effect on the volume and sound you have a+.
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.
