Yes, I get that with both of mine. I believe it is due to the bypassing of the overdrive stage since it only uses the booster stage which is similar to the original IIB. No compression so the booster stage will pump it up. Not sure if there are any changes to the first gain stage so that can also contribute to less clipping and more headroom. Not sure if there are any other changes in the pre FX volume or post FX volume. I found the IIB is much louder than the other two modes. Always have to trim back on the volume to compensate.
Mark IV mode also increases the sound a bit compared to the IIC. Similar to the difference between clean and fat. Not sure if this is just a bypass on a grid stopper resistor (I only reference that since the Mark V does that with the bold switch).
There is also a volume difference between crunch and VII modes on CH2 (That circuit pulls in the booster stage that follows the overdrive stage). This also increases the noise floor as well.
Sorry if this is a repeat. You can get all of this information from the tube task chart in the manual if that is easier for you to follow. However, the manual does not identify where the tone stack sits in the signal path with the crunch and VII modes. I emailed Mesa and asked the question, they stated it is post gain for the crunch and VII modes.
You can look at the tube task chart to see what stages are used for each mode. I made this for simplicity.
3B is the overdrive stage that has the 82k plate resistor. 4A is the booster stage that has the 270k plate resistor. I did not map out the clean modes. That would just be a total bypass on V3B and V4A for both clean and fat. I was more curious to see how the different modes having the most gain or distortion were routed.
I thought this graphic image was easier to follow than a spreadsheet listing the stages. This I created from the manuals of several Mark amps. The number in each column under the triode for each tube indicates position in the signal path. The red letters identify the lead drive circuits (overdrive and booster stage). Booster stage will always follow the overdrive stage (some modes will bypass one or the other). Both the Mark V90 and mini marks (25, 35) use the GEQ output circuit to create the FX send level. All of the other marks (not listed IIB, IIC+, III and IV) make use of a similar circuit used by the Mark VII and JP2C.
