So, Who Exactly is a Member of the Big Six?

It's almost as bizarre as Jethro Tull being classed as a Heavy Metal band at the Grammy Awards, although 53% of people on Ranker agreed with Dream Theater being classed as Neo Prog. 

There were Pink Floyd influences even on the debut album, especially Radioactive Toy. 

The music was progressive from the start, like you say, indie-psychedelia-space rock. Someone mixing genres like that, sounds like prog to me. 

True, nothing to do with neo-prog. 

Back in the early noughties, I used to personally refer to these as the 'Big 6' of the UK Neo scene..

Marillion - IQ - Pendragon - Arena - Jadis - Pallas

I doubt whether that would have meant anything to anyone else however, as Marillion had partly moved away in style and I'm sure other bands such as Landmarq, Galahad and Grey Lady Down might have had something to say on the matter... 

All six of those bands you mentioned are everything I DO like about Neo Prog. 

I never mentioned neo in my post.  I just wrote Super Seven, my super seven of prog of the post-classic era.  Shadowland nowadays are more darkwave than prog, closer to The Mission than Arena. As for Pallas and Pendragon, never liked them.  I don't recognise hard rock and metal as useful tags anymore, as Arena are in some songs just as heavy as Threshold.