Erik,
I recently built a room in the basement, in far less space than you have available. My room is about 13 x 20 x 7. How nice it would have been if I could increase the headroom! I love your 9 ft ceiling. Yet with careful speaker choice and placement, plus a good deal of acoustic treatment, I have been able to get excellent sound in my room.
Your width and length are both multiples of 7 ft. You might want to change that.
Also, I was told by one of the acoustics companies (maybe ASC) that they preferred width of at least 14' for best acoustics. My own experience would agree, in that my old, wider, room was easier to set up acoustically than this one.
Personally, I like a large room for audio, but perhaps not 44 feet long! I found this room calculator helpful:
http://www.bobgolds.com/Mode/RoomModes.htm Be aware that the effective room dimensions may not be the same as the interior dimensions, because bass will travel through the walls to some degree, much more than through the concrete of the foundation.
One warning: In a (partially) concrete-enclosed room, you will have strong bass nodes. I know that my room does. I have many large bass traps (which helps a lot), subwoofers to even out the bass, and I'm using some of the EQ in my Classe CP-800 preamp to help even it out more. Your larger dimensions might help some in that regard.
If you are in or near an urban area where you might be able to find a qualified acoustician to advise you, that might be worth checking out. I did hire one for a few hours to advise about ventilation and soundproofing. If there is mechanical equipment in the basement, you will need to consider that, too. Soundproof construction, however, will increase the issue of bass nodes. There is no free lunch!
I hope some of that helps a little.