Re: Quiet as possible.
Steel wool will BURN.
Don't use it.
To the OP- Experiment with adding an after muffler with some hose clamps and silicon or rubber tube and a mower muffler. You don't want to cut down on the flow too much, but you do want to increase the area of the muffler..
Also, quieting the intake will quiet the bike. What sound do you hear when you twist the throttle? On every bike I have built with the china in-frame it has been the intake "roar".