I have done something similar, but instead felt ring used low density foam about 5mm thick, without a cut out in the middle. It is not glued to the housing: when re-inserting the driver it squashes the foam a bit and keeps it in place. The results are worth the try, to say the least. Bass really comes out and mids sound more natural. I didn't see much impact on treble.
3/25/17 at 5:50pm