In the wording, it refers to making the power section akin. This would be intrepreted as using the same components, i.e, caps and resistors and values. The head room has a bit to do with the amps plate voltage and the available voltage on hand to produce loud, clear notes that do not sag as the power section strains to give the power amp section the voltage it needs to get the signal out. The MK III power transformer puts about 460 Volts to the plates. Under a decent tube draw they can read from 430 to 440 volts. The IIC+ 105 is rated in the schematic for 483 volts. On all three of mine under a decent tube draw they read around 480 volts which means they are probably putting out around 500 or so plate volts. The IIC+ has a much larger well of power to drink from and does not sag due to the power sections ability to provide voltage. You can audibly equate this to less sag and high head room, or the ability to keep notes clean and clear at high volumes. Now, the misnomer for high head room being a good thing is that amps that run lower plate voltages tend to sound sweeter. The MK III may run lower voltages, but the power section designs are both bordering on class B and are both obnoxiously loud.
( Class B citation: US patent # US4211893 ).
The upside of both are the MK III and IV tend to be a more well rounded amp for ultimately gettting to the sweet spot quicker. It does not suffer from having lower plate voltages, but gains from it. Post Blue Stripe they tend to be pretty loud but have a refined audiofile feel to them, much like the feel from a Soldano SLO 100 ( 500 Plate Volts as well) as they have the ability to give a sweet compression earlier on the Master. The IIC+ has this ability, but you have to know how to dial it in. It gives you more clarity and less compression, but has a well rounded and defined tone. For cinical settings it can go louder than the III, IV and maintain a sharper, focused tone with more shred to it but similar to the SLO 100 master settings below 4 do not show these qualities. Too many people who review the SLO 100 claim that is buzzy. With the design of the amp, it is a high headroom amp and you do not get the power tubes involved until you above 4 on the master and the amp turns into a super audiophile amps with tons of natural bottom end and no buzz from the high gain. With the IIC+, you have the same ability, but with more control on the lead volume. The louder the master settings the more you get the whole amp to integrate and blend all of its individual components together. IMO, you get the these benefits at and earlier, more managable volume with the III and IV. Don't forget that these qualities are further refined in the MK IV that runs lower plate voltages to the preamp tubes in R1 and R2. So, to finalize, I would not get caught up int he head room debate on the MK series amps. They are what they are and all offer something slightly unique over there earlier or later lineage. I feel that the IIC+ has gained the title of "Holy Grail" for it's ability to have a unique tone unto itself. For some, having the qualities that it does make it the right amp for them, but for others the III or IV may be better. I feel the mystique of the IIC+ is warranted in it's tone, design and at least it's limited production. In essence, it's Boogie's Dumble. People also have to understand that when you use different circuit boards, tranformers, capacitors, resistors, layout and voltages you may end up with an amp that is designed around a superior amp, but loses somthing along the way.
A good case in point would be Marshall. You don't have to have a super tuned ear to tell that a 67' plexi sounds superior to a 2006 re-issue. Although, Marshall has been having it's lunch eaten by companies that paid much close attention to the qualities that made the original plexi's and JTM 45's superior. Germino, Blockhead and a host of other companies have done pretty well at selling point to point Marshall copies over the years for relativley decent prices. Then after years of losing market share Marshall come out with the hand wired product for unreal prices. I hate to say that they still did not get it right and Germino is still a better sounding amp at half the price. Mesa has stated many times that they will not reissue the IIC+, although it could be made and sold at competetive prices and actully very profitable. They firmly beleive that it's essence is captures in the III and IV. They are the lineage and are all we have to enjoy except for those few IIC+'s in existence. As I have stated many times, you would be lucky to have three IIC+'s that sound the same yet alone trying to compare a single C+ aginst a III or IV. They are close, but not exactly the original. The same DNA, but offspring with there own personalities. For those who want a C+, it's time to shell out. For those who have a III or IV, play the amp and enjoy it. It's gotten to the point where it has made people second guess there decisiion and hate a perfectly good, and sometime phenominally sounding MK III or IV because they are in the shadow of a phenominon. It's like the Manning family. Archie was great, but played for bad teams. Peyton is already a Hall of Famer, but has not one the big one and Eli, well, who the hell knows about Eli. All the same DNA has proved they have a family trait, but are all different.
Time for a smoke and a pancake.
