I wouldn’t use a plexi to get the Laney Supergroup sound, they are pretty different in the low end.
I have the Laney LA Studio Supergroup, the 3 watt head with built in two notes cab IR. Pair it with the Black Country Iommi boost pedal and it’s 99.9% spot on early Sabbath tone. Awesome looking amp too with the retro knobs and white panel…..
Anyways, Marshall’s in my experience don’t have the low end of a Laney. My SV20 for example has way more upper mids, the typical Marshall “kerrang”, but doesn’t sound like Sabbath
Now Geezer is a huge part of Iommi’s tone, but the Laney sound is too. Those Supergroup amps are very vintage sounding, and have a really big bottom. When you hit them with a treble booster, which they 100% need as even dimed they don’t sound like Sabbath, they really come alive and you get that big but not totally distorted Sabbath sound. It’s not all that dirty, it’s not very tight, but it’s THAT sound.
Given we don’t have a Supergroup, I’d try something like a JTM and of course treble boost. Your not getting the tone from the amp alone so plexis and JCMs etc are going to nail it, Bassman, JTM45, etc are where I’d start
Or get the little Laney LA Studio and run the line out into the Axe, then you’ve got Iommi’s exact rig lol