My opinion is the that all the full size trials bikes compare equally in decibel level. The stock silencers are good, as long as they have good packing. On a full size bike, most riders average club riders are not revving their bikes all that much. I used to routinely ride in my yard, when I lived in town and my trials bike made less noise than a lawnmower.
From what I've seen, it's the small bikes, 80cc and under that are obnoxious. I don't know why this is, but the kids riding around on the little bikes are louder than the regular trials bikes. My guess is any of the bigger bikes are going to be quieter than yours, unless their muffler is in need of repacking.