Matt, that is all very interesting. You may be right about low/mid stuff passing right through the box, hence the "middy" / "high end" sound to the recorded result. It does also sound "boxy" (no pun intended), I wonder if it was more "breathable" if this result would diminish?
In looking at some of these reflexion filters online, some (or most) of them seem to have some sort of perforated metal at the very rear, and padding in front (on the inside facing the mic) to minimize reflections. Is this so the low end information DOES pass through? I'm thinking of the construction of my bass traps, that they are designed to allow (absorbtion) of low end frequencies by enabling them to pass through by their perforated, breathable backs. However, as Jeff pointed out, it would change your tone, especially if you were a Baritone, eh?
I was thinking of almost using some sort of plastic bucket, cut in half, design but this would prevent sound from passing through. Would this be a bad thing, even with foam lining the inside?
Thanks for the insight Matt!