Hey there. So S3RL has stated before in an interview from 2014 that he doesn't use external VSTs in Reason. Therefore he doesn't use Sylenth at all. His sound has changed since 2014 though, but he likely still does more or less the same techniques. He has typically just used Subtractor and Scream 4 to generate his sounds. Subtractor is a subtractive synthesizer native to Reason, but you could use Synth1 or Sylenth as they are the same idea.