the Sl+ has alot of gain, perhaps adding some lower gain tubes might help tame it and get to that sound, but I think it may be a different overall sound.
I don't think many people turn the gain on their SL+s below 3:00 tbh But yes, turn the gain down to between 9-11:00 and you get great low-medium gain tones reminiscent of the Plexi. Even though the bass cap is lower than that of the Plexi, I still turn the gain to around noon, crank the mids and highs and get a great classic rock-ish tone. Use the volume knob with moderate to low output pickups and you have a nice spanky clean too on the lower gain settings
If you're like me and just need that dirty Plexi vibe occassionally, then the SL+ can handle that tone..but if you need that Plexi vibe as an integral part of your sound, get the Plexi/Brahama
I run my SL+ with gain at noon. It's spot on for the JCM 800 there. The only module I have that I run at 3:00 is the Grail. But then I run my MV at 3:00 with a Hot Plate....
check out imsjr on youtube. He pits a sl+ against a jcm800 and talks about the soldano tone after cranking the gain. good video to see. search sl+ vs jcm800. As far as the plexi tones they are close with the gain low on the sl but the clarity of the plexi is not there with the sl+. I have really tried to convince myself that I don't need both but then I play a les paul with the neck pickup or a friends p90's through the Plexi and cannot let it go. It is similar but the 2 have differences.