Again, my construction knowledge is a decade old... but I'd be inclined to use a spray-in insulation - either blowing wool (fibreglas strands), cellulose or possibly polyurethane foam - this stuff was just coming in as I was deciding construction wasn't for me - ie: the spray-in expanding foam (fibreglas bats are fast and cheap, but not very efficient, at either dampening or insulating).
Down from there I'd probably want to seal in the "dead cavity" with 5/8" fireguard drywall - perhaps with RC or furring between it and the joists? - at this point, you've got a space where bass goes to die... problem is, the drywall on the bottom is now acting as a drum skin... from there, I'd use RC between that layer of drywall and a second layer of 5/8" to minimize the amount of vibration transfered to the actual ceiling.
Having two layers of drywall like that shouldn't be too much more expensive, since the top layer doesn't have to be finished.
Used a similar technique for a condo wall at a buddy's place... before the treatment, he was well aware of his neighbours', er... libidos. Afterward, he heard nothing at all (which, after his wife left him, may have been a drawback, instead of a bonus!
lost a cheap form of entertainment!)
Just my two cents, anyway.
Bren R.
