I've got a Epi Custom Shop SG that I've completely upgraded, which included a set of Riff Raffs. I'm in a Bon Scott era AC/DC covers band so I have a similar tone to the one you're after (70s rock). I also have a Les Paul Std with 490R/498T pickups so I know where you're coming from re. muddy tone.
My SG has so much more clarity than my Les Paul. The LP obviously has a thicker tone anyway, but it seems to be "mud and fizz". In comparison, the SG is "clear and bite". On the neck pickup, the SG just sings bluesy licks. Stick it in the middle position, roll back the treble volume and both tone pots a touch, and hit the "Red House".
Sorry for the long winded answer, but what I'm saying is:
* Go for low output rock-y pickups: Riff Raffs, Mules, Mississippi Queens. Maybe an MQ neck and Riff Raff bridge if you fancy something versatile?
* Lower the pre-amp gain on your Marshall and boost the output gain to get the EL34s cooking. Don't use too much fizzy front end gain.
To get that true 70s sound you need a cranked amp with low input pickups.