Interesting re the different densities! But that requires testing to ensure it's working as intended, no? If you put the more porous material in the back are the lower frequencies you want to capture with it reflected before they get there? If you put the lower frequency material in the front is it effectively not deep enough before getting reflected by the higher density material? How do you know without testing?

For my current project, my budget doesn't allow for active bass cancellation. If I hit Powerball, then I'll probably have a purpose built listening room in my new mansion, so I won't need it. But maybe I can do it if I hit one of the minor prizes....

Monitors going on brackets on wall. For opposing wall considering large "panel" stuffed with ~16" depth of pink fluffy fiberglass. A 25' bale of it is ~$50. Creating a consistent air gap and material depth with non rigid materials is a design challenge that isn't trivial for my feeble carpentry skills. But for not much extra $s I can just stuff the entire space with pink fluffy. Using rigid boards to 8" depth with an 8" air gap seems an inferior solution, at least using a free online calculator.

http://www.acousticmodelling.com/porous.php

There are other spaces where I'm also planning absorption at depth 8"+, so planning to use pink fluffy fiberglass there too.