For absorption, OC 703 panels behind and between speakers. I've purchased from acoustimac.com with excellent results. Sal's prices are very good and shipping is inexpensive.
It seems as if the OC 703 panels are similar to the ATS panels at a lower cost. Is the ATS a similar fiberglass material, but just already covered in fabric.
Assuming your speakers radiate forwards then anything thicker than about 4 inches will be worthwhile for broadband bass absorption. I would not bother with midrange or treble as nothing much over 600 Hz goes backwards behind the speakers so anything thinner is not likely to have much effect.
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