dude, did you ever check the link i provided?
it's got most all of it.........
http://homepage.mac.com/mesaboogie/MarkIIC.html
"The Mark IIC+ was the last of the Mark II series and featured a more sensitive (i.e. useful) lead channel and, more importantly, an improved circuitry in the effects loop. Unlike earlier Mark II models, pedals could be used without the amp's signal overloading their inputs. However, the volume pedal option on the Mark IIB discussed by Sacks cannot be implemented on Mark IIC+s.
Some owners/dealers/sellers often say the "+" refers to an amp having an EQ, but they are mistaken. The mistake originates because in the mid 1980s Mesa 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.
You 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. Be aware that many vintage dealers jack up 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, so let the buyer beware. But, you can call Mesa/Boogie and ask them to check your serial number against their records. Mesa/Boogie only made about 1,400 Mark II amps before going to
the Mark IIC+."
and this:
"Simul-Class/Class A Switch: In the Simul-Class amplifiers the power (before clip) is much less in the Class A position - mere 15 watts. But the unique way the system is wired allows you to achieve a much greater degree of power tube distortion than with most other amplifiers ... and it is still very loud! In Class A, only the outside (outer left & outer right) pair of tubes - the EL-34's will be on. The inner pair - which are 6L6's - will also be turned on when the switch is in the Simul-Class position. In addition to this obvious difference, the biasing and control circuitry of the socket pairs are radically different ... and this is the heart of the now famous (and patented) Simul-Class circuitry."