I'm pretty sure it was Hydro Power Inc. When our boat was being made I called up these guys and put them in contact with Wakecraft and since they did business with several of the other manufacturers in the area they dropped by and took a look at the engine and spec's of the PCM ZR6 and took it from there. The first set was too quiet and I think a little restrictive but the second set was it. Not too loud and not too quiet. Gave the boat a great sound IMO. You can hear it on Youtube just type in Wakecraft and you will find a low quality video that I did just for the sound. The site is
www.hpisilencers.com found this on Waterski Mag
http://www.waterskimag.com/article.jsp?ID=42113
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