I like that a ghost knob.
I went throught a fad a few years ago of having to buy almost every Mesa amp made(as it seemed). I did learn some things about tube amps and there EQ controls. After a certain point on the gain knob Eq just don't matter as much. They don't work because the tube is saturated beyond the larger control of the Eq adjustment. More so with higher Freq. if I remember but gain can also effect the bass as well.
What I found is try to control the gain not to effect the eq as much and find that sweet spot with the gain and the eq and the presence.
Another way to look at it is each control acts as a gain control but in a different frequency range. Keep in mind this is coming from using Mesa amps and I do the same with my fender rigs but always found differences with Marshalls.
The presence control seems higher and Marshall amps are very strong in the mids and high frequencies.
I have owned two different Marshall tube amps in my life so please be kind. The first one was a JCM 2000 401 and the other was a DSl 50. To be honest I can't remember what happend to the Dsl 50 for the life of me. But the 401 was sent to me with the reverb can laying loose against the power tubes so I returned it right away.
I did play on a JCM 800 along time ago and didn't like it. Everyone says there are the best and I think perhaps it was just not set up well when I played on it. I have learned a few things since then about tube amps and how to set the gains ... So I don't have much time on Marshall amps at all come to think of it. Mostly Fender or Mesa never really like Peavey. Always like Marshall sound Just havn't felt them out yet.