I played for years with an acoustic guitarist/singer-songwriter who used to routinely, specifically, ask for more 100 Hz when we soundchecked. When he strummed, he also used to mute a lot with his picking hand for a big, chunky sound. He played solo gigs, and all this would work just fine in that situation, but it made me disappear in the mix when the two of us played together. Finally, we happened to have a discussion with a soundman who often worked with James Taylor. It turns out that James often asked for the same thing (boosting around 100 Hz), and the soundman would politely find a way to refuse. That finally convinced my friend.
Anyway, the longer this thread goes on, the more it sounds like the problem lies with bandmates who don't understand the concept of tailoring their personal EQ and volume to make the whole thing sound clear.