Welcome!
In case you didn't notice, there is actually a special section on the forum for "Music and Theater posters". It doesn't see a whole lot of action compared to the movie poster section, but I think you'll find some fellow enthusiasts.
As for myself, I have a few posters I've randomly picked up over the years for musical artists I like (Elvis Costello, XTC, etc), but I'm sorry to say I don't know anything about the large subway posters you mentioned.