Again, I don’t mean that you just get extra thump (actually, it is a little easier to limit, as the powered sub will probably have a dial on it to control its output independently), you’ll get richer, clearer, more proper sound with a better setup, and possibly better speakers… At the very least, you’d need a new subwoofer and a new AVR, you can wait on the speakers if you must as long as you get an AVR that supports the impedance (though, many subs will run you as much as the full Onkyo set I linked to earlier). If you’re not getting rid of the system, I think you already know you can just plug the current player/receiver into the new receiver, it should passthrough over HDMI just fine.