In a word, lots. Admittedly, I’m a recent Marshall convert, having been primarily a Fender guy for years. I blame Jeff Aragaki, owner of Aracom Amps for this. His amps, that are steeped in Marshall mojo got me craving for that type of sound. BUT, it wasn’t until I got the PLX18 BB, a Plexi “Bluesbreaker” replica that I “got” that Plexi vibe.
At least to me, the response I get is visceral. When I hear a cranked up Plexi, there’s something that just sparks within me. I literally sink into the tone! A bit dramatic? Probably, but to me, there’s nothing that defines classic rock more than a cranked Plexi. Here’s a video I found of some classic Cream with Eric Clapton, playing “Sunshine of Your Love:”
Mm, mm, good….
Guitar Gear 
What a great classic sound! Eric at his best. Gotta love the fur outfit and Ginger Baker’s very strange drumming style.
Nothing like a great 3 piece band! PAF equipped guitar plugged into a Plexi = Fantastic Tone.