It is worth bearing in mind that all of these 500/550s sound rough at low revs even when they are set up correctly and all parts in good order. I had a genuine 7000 mile engine with replacement cam chain, tensioners etc and it sounded like all the others I have heard, i.e. like a bag of nails, but increase the revs slightly and all extremely smooth. 17 years later and it sounds exactly the same, no worse.
I don't think they are really designed to run at low revs. When I adjust the tickover there seems to be a sweet spot which I run up to. My rev counter is broken but I think it is about 1200 revs. Just remember, if you have the tickover too slow you won't have as much oil going up to the top ;-)
(maybe you can get them to run slower with stock airbox, exhaust and electronic ignition)