Can I run Cubase 6 if I have a Cubase 9 license?
yes
Depends on what exact version the “Cubase 9” License is.
I’m talking about a normal Cubase 9 Pro license.
My Cubase 9 Pro license can run Cubase 7~9(I only tried those three version),
I thought because I brought those three before, so actually I can run all Cubase version before Cubase 9?
Then, as Jaslan said: Yes.