I would also advise - hunt down some isolated multi-tracks, or (as I did recently) - refresh yourself with a Youtube concert from the late 80s, and listen to the guitar in isolation.
There is much, much less bass than you'd think - the JCM800 is not a bass heavy amp, but put it with a band, and play it loud - amazing.
I'm talking about a different age, I didn't play much at home and accepted that when I did it would sound mushy - but when I took the stack to rehearsal - we rehearsed loud, proper hearing loss, ears ringing volumes. And at that level with a bass, drums etc. the JCM/Jubilee - for the music of the era, absolutely ruled.
But listen, at home to those isolated multi-tracks, or (as I did the other day) - watch a live video when the guitarist hits a few chords just checking tuning between songs, and you'll notice - no bass at all, none.
As others have said, Marshalls can sound bassy - couple of years ago, I had a 54 Goldtop, loved that guitar. Played a friend's Plexi - was very loud but sounded great, we smiled and cranked it all the way. Honestly - didn't sound so good, like a bad fuzz - because so much distortion we needed to turn the bass right down, and truth is no one played that loud - look at the classic stuff, it would be loud but then with a Super Distortion or a Distortion+ in front for more gain, and those pedals all cut bass.
Gary Moore with his JCM800s ran a Boss DS-1 in front, sounded amazing - now we'd sneer at that.
Sorry random Sunday thoughts!