I'd suggest reading "Sound Studio Construction on a Budget" by F. Alton Everest. He covers domestic listening rooms too and this book came 10 years after his "Master Handbook of Acoustics", and has info on newer products--RPG, especially. You can learn all kinds of things about room acoustics from this book, and where to locate your four absorptive panels in particular. You'll probably want to do other things too.
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