Owens Corning 703 vs. Roxul Safe 'n' Sound?

At $47 for a 12-pack, you can just stack them up in the back of the room to see how it will affect the sound. They 15-1/4" are cut to fit between house frames. If you don’t mind putting up 3 panels instead of 1 or 2, it can be a cheap way to go. The Roxul are only 2.5 lb density, but they might be stiff enough to lay up against the walls. If you do this, be very careful. It is mineral wool fiberglass and always use a filter mouth mask and gloves when handling them.  There's a point where there will be too much absorption in the room.  You'll just have to experiment until you find that threshold.

The panels from ATS Acoustics are mounted in a light wood frame and wrapped with fabric. At $55-76 per panel, it’s really cheap for the build labor. Unless you are fine building them yourself.

Nah, I’ll make one frame sized to fit three pieces of the Roxul in, 45-3/4" x 48" interior. There is 56" between the two doors that flank the listening position, on the wall opposite the speakers. I might put a Roxul strip on each of those doors too, if they prove to be reflecting too much sound over to the lp. The space between the doors and the side walls is far enough away from the lp to be of no concern to me. That’s all the absorption I want---just to kill the first reflection from the speakers pointed at the lp wall.

The room sounds fine overall, no glaring problems as far as I can tell. I have ASC Tube Traps and a DSPeaker Anti-Mode for low-frequency eigenmodes, and MG CornerTunes and EchoTunes for any slap and flutter echo emanating from up near the ceiling. The wall behind the speakers will remain reflective---fine (in fact, preferred) for dipole speakers, as will the sidewalls---no need, due to dipole cancellation. First, I’m painting the room, this weekend, hopefully. I chose Sherwin Williams "Mink", a combination of gray and brown that is absolutely beautiful!

One thing to know about the acoustic materials is that you need massive amounts of thickness if you want to absorb a lot of the lower bass frequencies. Yes, putting a space behind the fiberglass will help because the sound will bounce off the wall and back into the fiberglass. However, if you want to absorb the lower frequencies, you’ll need a very thick trap (like the 6-7" bass traps from GIK). If you want to put multiple layers of the fiberglass in, you need to keep them at the lower densities (like 2.5 - 3 lbs).

I chose the 2" thick 8 lb Roxul because I wanted to limit how much mid/high frequencies it would absorb, but I still wanted decent bass absorption. At a certain point, the denser material will start reflecting sound at a thickness level. A 4" stack of the 8lb Roxul will probably not work any better than a 2" stack. However, putting a 4-8" stack of 2.5lb Safe-n-sound will absorb a lot more bass (as well as a lot more mids/highs).