This is an interesting topic!
I think I first heard about Magma in the late 1990s, through Paul Stump's book "The Music's All That Matters: A History of Progressive Rock". At that time, I was looking to find out more about progressive rock. I already had a life-long love of Yes and Rush, and the music of King Crimson was basically re-wiring my brain at that point. But otherwise I didn't know all that much about this type of music.
In Stump's book, Bill Bruford is quoted as saying:
"Magma, though, turned heads; they appeared to be an elemental force that was completely unlike anything else."
To me, Bill Bruford basically sounds like Jesus on drums, so I took his quote in the book seriously. Add to that this (non-Bruford) quote I found shortly thereafter (I found it again on http://www.geocities.com/
"Very powerful stuff. On the live album, there is one 20 minute epic 'Kohntarkosz' which builds up to an *absolutely* terrifying climax. You thought Yes built up 'Close To The Edge'? You think King Crimson built up 'Starless'? You've heard nothing yet. Just when you think it can't get any more tense, they give it another twist."
I found the references to Close to the Edge and Starless enticing, to say the least.
Finally, I think some of Chris Cutler's descriptions on the RER website really convinced me that I had to hear Magma. (Sadly, it looks like some of those description are no longer on the RER website.)
Because I had no money at the time, I convinced a friend to buy some Magma... and he got MDK and the Magma Live album. For whatever reason, the Live album didn't impress me much, but MDK was just so weird and different and heavy and life-changing that it really "did it" for me.
...And I have been a fan ever since. I even bought the MDK score later and taught myself (by rote) to play the first seven minutes or so of MDK (I don't actually know how to play the piano).