The Cornwalls would tend to dominate a space of that size and would be limited in their ability to perform at their best, not having sufficient breathing room.
It would seem to be a rather "live" space from your construction details.
Room treatments are always a "best buy" as they will stay with you throughout and hardware changes.
If speakers are in your future, twoleftears has provided some good reccomendations. Also add Harbeth to that list.
Might be worth while to look into standmounts/monitors for that size room.