A little cut & paste from another post...
"I got my SL+ from vindaloo yesterday. He did the JCM 800 mod on it, and while I've never owned a JCM 800, it definitely has that flavor. Probably a little "gainier", but otherwise pretty close. I screwed around with it quite a while, and found that the best sound I was getting was when I had the gain at about 10:00-11:00, and hit it with a Boss Super OD, with the gain at 9:00, and the level at 3:00. WOW! I did the same thing with the Plexi, and also got great results. I tried to fins a sweet spot with my MXR CAE Clean Boost/OD, but at the end of the day, the $50 Boss pedal sounded better to me. Al bit noisier, but it nialed 80's tone like nothing I've owned before.
As far as the SL+ vs. Mr Scary - very different voicings, yet still similar. Odd description for sure, but very true. The Mr Scary is darker and a bit more dynamic, but lacking the low end the SL+ has. I dial back the low end on all my modules though - I don't like my amp to "thud". I find bass-heavy EQ to just seem flubby, no matter how "tight" you perceive it to be. I try to find my power in the mids."
One other observation - I would say that the gain in the Mr Scary is a bit "smoother" - almost Mesa-ish, where the SL+ is more "grainy" and crunchy.