Convert from more mainstream to more obscure prog?

In my undying effort to convert people to the ways of prog I have noticed that plenty of people like Pink Floyd proto-prog and prog-related music along with some of the heavy prog bands like Rush, Mars Volta and Porcupine Tree but are unwelcoming to some of the music I give them to sample. How would you go about weaning some people onto the bigger prog scene? For any of the bands below please provide a chain like The Doors ->???? ->???? -> King Crimson

Pink Floyd
Rush
Porcupine Tree
The Mars Volta
Led Zeppelin
The Who
The Doors
The Beatles
Any other bands that were/are considered "mainstream"

It's best to think about what the person likes now, and find a gradual thing. With my very metally friend (understatement), I have been attempting to get him onto more proggy things, so starting with his already present love of Death Metal, it lead to Opeth and Meshuggah. This then led to Ephel Duath, who I would say are more obscure and leaning more towards the jazz side of the spectrum. I hope to eventually get him onto Zappa  He liked some of the Mars Volta's "Bedlam in Goliath", so I think as long as it's heavy and not uplifting, it may well appeal. Next stop: King Crimson!

So this go would go something like Death Metal in general -->Opeth/Meshuggah-->Ephel Duath-->the Mars Volta-->King Crimson(???)