I don't have a plexitone and know with it and a 1987x you can get 80s levels of gain. Any OD type pedal will get it there unless you are running the amp clean.
One thing to note though, pedals will perform differently in front a plexi set clean and a plexi set dirty. The more the amp is cranked, the more it's own character will be there and less of the character of the pedal.
Ok. Let me clarify... I like setting the amp just at the edge of breakup. I use the CM to get 70s tones. And it absolutely nails them. I'm thinking about another dirt pedal for more gain and to leave the CM where I have it at.
Just crank up the gain on the CM for heavier tones and roll your volume down on the guitar for cleaner tones.
That's what it's there for.
It's the beauty of running an OD into a Plexi with a good guitar.
I've been doing it for years.
To get the 80's tone try a Boss SD-1, it's even better with the Monte Allums GT mod. I was on this quest for my 2204 last year and this nails it. Besides, it's so cheap you have to give it a try.
+2 on the bogner red. The suhr riot is another good bet.
A for the mi audio crunchbox, they have a super crunchbox out now, which from reviews it sounds like they address all the things I didn't like about the normal crunchbox.