Here's a very unscientific test. Lot's of room for human error, slight difference in patches etc but this audio file shows DUNE 2 first and Sylenth second. Both set as closely as possible to the same patch. I'd say it's at least worth having Rich take a look at the issue and see what he says.....
Dune and Sylenth Mod Test
By the way how to do get audio files to play here without them being on soundcloud ?