As another data point here, I just removed my Hushpower II, and it sounds like a junkyard inside.  It is basically a hollow shell with loose metal trapped inside.  This has 250 hours on a NA 13B with splitters in the exhaust ports.

Bill Eslick
www.weslick.com/RV6Index.htm

On Fri, Jan 16, 2009 at 4:12 PM, Tracy Crook <tracy@rotaryaviation.com> wrote:

<snip>
The Hushpower II looses some but not all of it's effectiveness as it
ages (loosing packing probably) so I'm still searching for the optimum
muffler.  You might be on to something here.  Wish it was available
off the shelf.

Tracy