So you’re saying you are putting the fiberboard on the outside of the wall behind the speakers? If so that makes sense, sorry if I misunderstood it sounded like you wanted to put it inside the false wall which would help dampen outside sound some but not do much for interior reflections.
I’ve read and found from testing that if I keep the drivers of my M80s at least 3 1/2 feet or more from the wall the reflections don’t have any negative impact on the soundstage. OTHO putting temporary absorptive material at the first reflection points actually shrunk the soundstage making it seem less wide.
However if I put the speakers closer than about 3 1/2 feet from the wall, absorbing the first reflections improves the front soundstage. I’m assuming from your post you won’t be able to pull your speakers that far out.
1) if I do the bass traps in the corners then the speakers will be pretty darn close to them. Which is worse no bass traps or no space behind the speakers?
Well if having bass traps in the corners means no space behind your speakers then it sounds like your speakers will be “corner loaded” which will usually colour certain frequencies and reinforce some of the bass quite likely in ways you don’t want. So I would probably use bass traps up front as long as you can have a few inches between them and the rear ports on the speakers.
Not sure if your room is symetrical but if the front and back corners are similar you could make a pair for the back and also try them up front to see what effect that has on the speakers.
Personally I wouldn’t do everything all at once. I did that in my apartment and found out I did much more work than I needed to because I was going of theory and not listening to what was actually happening to the sound in my room. That’s why this time around I’ve spent 4 months moving speakers, furniture, wall coverings etc… around to see what effects it’s having before committing to anything. I’m very happy I have because I’ve already found I don’t need to absorb first reflections off my walls. I don’t need to build a full wall up front just a wooden frame to hang the screen on so the aluminum frame won’t sag. And that I don’t want thick fluffy furniture because it sucks the life out of the room (just killed the ambience). So before building a bunch of stuff you might want to start listening to your system moving and tweaking it as best you can. Perhaps use some temporary sound treatments (pillows, even mattresses can work) and see what effects you hear before deciding what will work best.
2) What material are you guys using up front around your screen? I guess the black material around the screen doesn't need to pass the "breathing test" -- you agree? If it does then how do you cover up behind so you don't see right through?
Right now I’m planning to use this for around my screen, it’s probably overkill but I want no reflected light other than my screen in my field of view. http://www.fpi-protostar.com/flock.htm
If by “pass the breathing test” you mean acoustically transparent (AT) because you are putting your speakers behind the screen then yes you probably want it to also be AT material. However, if your speakers are going in front of the screen I wouldn’t worry about it.