How do you treat a room to make it ideal for mixing and listening? Do
you ideally want to stop ALL reflection and reverberation? Ie a
completely dead room, or do you just want to tone it down a little?

I have a room in my basement that has 2 poured concrete walls on the
left and back, drywall on the front, and wide open to a large rec-room
on the right. The floor is concrete, and the ceiling is about 9 feet
high, with exposed floor joists supporting the 1st floor.

What are some simple things I can do to make this room as good as
possible? Carpet? Frame/drywall over the concrete walls? Put in a
drop ceiling, drywall ceiling, other? I heard parallel walls are bad,
they bounce sound back and forth or something? I am planning on
finishing the basement, so any suggestions would be great. Thanks,