Miklos,
I've run into similar problems with the Slimline brand as well as the ones I've made myself. What I like to do is get some door screen material (the metal kind-stainless preferably) and mold a few layers (2-3) around the tip of my finger. Repeat process again molding the screen small enough to snugly fit into the exhaust tubes. I then take a pin drill (1/32" should be ok) and drill a hole clean through the exhaust tubes about 3/4" from the exit openings. Insert the molded screen "cups" into the exhaust tubes and pin into place with a pin from the wife's sewing kit. Push the pin through, bend over the excess, snip off point. Done.
This will increase the back pressure just enough and take the crackle out of the exhaust sound.
I'll try to get some pics posted.

I pulled out my Spit when I got home. Here are some pics showing a nice neat way to do it. This muffler works great!



I suppose you could 'cap' the outside of the tubes with the screen but that might look a bit hokey.