Mark IIA-B Gain Boost VS Mark IIC-III Pull Deep

[Mark3Boogie]

What is the difference between the two things?
Someone say that the Gain Boost in the Mark IIA-B bypass the tone stack for more gain like the Mark I one, other say that is just a bass boost like the Pull Deep.
What is the truth?

 

[loylo69]

Someone say that the Gain Boost in the Mark IIA-B bypass the tone stack for more gain like the Mark I one

That is true.
From the Mark IIC, the gain boost / pull deep function adds a bigger cap on the cathode of the last preamp tube, resulting in more gain on the bass / low mid frequencies.