To seal or not to seal ...

To be constructive (for a change) your bass problems will be very severe, and bass problems cannot be cured by room treatment. The electronic approach is one that I use with great success, but in a room where the sound field is so resonant and stablized the electronic results will vary greatly depending on where exactly you place the the microphone. One thing that might help this would be a planar speaker, like a Magneplanar. The large area of the sound source makes the resonant sound field less sharply defined, and you might be able to do something with a Behringer DEQ2496.

High frequencies can be absorbed by lots of soft stuff on the walls and a rug on the floor. You might also want to hang something soft from the high plywood ceiling.

Unless the sound escaping from the 6" opening will disturb someone I'd leave it there. It seems that it would be good if you could get rid of some of the reflections in that room.

A square room is a problem as it will magnify the room modes. This will make the bass boomy and affect the midrange clarity.

You need to research room treatment. Bass traps are a BIG necessity in a square room. There's an acoustics circle at audiocircle.com to ask questions. Room acoustics play a huge roll in your sound quality.

Another option besides room treatment would be electronic room processing done in the digital domain. A Behringer DEQ 2496 will help the room treatment significantly. And again in a square room you need all the help you can get. Bass traps alone won't cut it.

Please do research on room acoustics. It's much more important than anything besides speakers, period. Acoustics can make an expensive system sound awful and a cheap system sound better than anyone would expect.

Good Luck

Hey before you seal the wall,let me send my wife for a quick visit!

Besides the already mentioned negatives, it's also an almost square room.
Try headphones rather than speakers.

Your room, as you describe it, sounds ideal for incarcerating terrorists.

Acoustics aside, make sure you leave a door so you can exit.

#1- the Tile will kill you, and concrete walls can be even more painful.. Better Look into really doing some drywall or at least cheap hanging and laying carpets on some of those surfaces and see how it will all sound first.. The opening can't hurt too much, might help a bit depending on the actual speakers and the response in that room. Fully sealed is not completly necessary unless you have Sound proff reasons to reall isolate it from other areas and driving people crazy.