As an fyi, anytime you go from larger ducts to smaller ducts you increase the air velocity, so splitting an 8 into two sixes will increase the velocity and face pressure. Moving from an 8 to a 10, for example, will decrease the velocity and face pressure.
It is possible to get the CFM you need with a low-profile rectangular duct that will fit inside your soffit. The flat could be used with the in-line fan by terminating the duct into a pressure box and having the fan mounted in the opposite side of the pressure box to create the negative air pressure draw you need.
I can't recall right now - but I think I remember you said your HVAC company would be willing to come out and "make you comfortable" if there was any problem with the basement temp. vs. the rest of the house. Have you talked over your options with them?
The other thing I will mention is that since your supply lines are connected to the main system with the thermostat upstairs, have you considered how you will avoid heating the theater if the upstairs floors are calling for heat? I realize in Alabama the overwhelming majority of the time you will be cooling, but there are probably several months where you will also be heating I would guess.
Perhaps a ducted split system is in your future.....