Guitar Hacker - My JBLs have a mic system for tuning the room similar to the ARC you mentioned. Helps close the gaps on my room problems. I have my room treated with some old Auralex 24 x 24 tiles I inherited from a friend remodelling his studio. I used the "Mixing Secrets for the Small Studio" book as far as where I placed the tiles around the room. Helped a lot!
Mixing Secrets is a MUST HAVE book for anyone serious about getting the most out of their home studio.
A sub isn't absolutely necessary. AS long as one learns and understands their existing speakers, a good solid mix can be had on just about all of them.
I try NOT to do my primary mix in cans. BUT... I do use cans from time to time to check the mix.
The most important thing here in this conversation is the knowledge one has regarding the strengths and weaknesses of the speakers they are using, and also a knowledge of how the room is affecting them helps.