Acoustic treatment

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With regard to the OP's question, I understand that Rockboard 60 is highly regarded. I didn't try it because the minimum order in my neck of the woods was a pallet.

I have some extra beer bottles.....

On the off chance nobody mentioned it already one key element of Tube Traps, the real ones, is that there is a reflective side that should face the room. This is also true for Michael Green’s corner tunes. Coincidence? In the immortal words of Acoustic Revive regarding the dodgy subject of damping, “One should be careful or else over-dumping can occur.” The other trick is to determine the actual standing wave location in room corners which may or may not be in the actual corner per se.

Quite right, GK. If you score some of the old Tube Traps (before ASC starting marking them) as I did (a guy advertised them in The Recyler in L.A. in the 90's---ten bucks apiece, for 13 of 'em, 9", 11", 16"!), the seam side is the absorptive side, directly across the tube the reflective. By the way, the Room Tune is just a piece of fiberglass insulation covered in grill cloth, nothing magical (unlike the ASC Tube Trap, which is a relatively highly-engineered product).