You're welcome! I think Marshalls are sorta underrated around here.
For what it's worth: I A/Bed a '76 JMP 2204 (essentially the same as a JCM 800, just in a sexier headshell) against my TSL when I owned both. The voicing is broadly similar- both are instantly recognizable as "Marshall" but there's a few differences between the 2000 series and the 2203/2204s. The older Marshalls are significantly brighter- or at least mine was. (Admittedly, it was a pristine stock 2204 that was pushing 30 years old when I got it. Probably needed new caps.) However, they are also tighter and clearer; there's a noticeable difference in the low mids. They're more "open" sounding as well; I feel they sound "bigger" than the 2000s, which I attribute to their less compressed feel, even when boosted into high gain. The older Marshalls have less bass on tap, but it seems to me the bass control affects a different, deeper frequency. So there's a bigger bottom end, but less of it, if that makes any sense. I tended to run the bass very high on both those amps- and I'm not big on having huge low end in my guitar tone. (I hate modern mode on Rectos; I can't dial out enough boom!) It's perfectly adequate with a band or in a mix. I think that's where these sort of amps show their true worth.
One thing about the 2204: it was the loudest thing I've ever owned, by far. Giggable volume with the master on 2. Not 2 o'clock, 2, as in 2 of 10. I usually ran my TSL on 6. Green/crunch should get you closest to an 800 tone on a DSL.
As far as the 6550s or KT88s, I'm not sure. You'd probably be able to get within the right ma range for the bias, but I don't know about the plate voltage and such. You could do it, but I don't know if you'd have to mod it to get them to run acceptably. KT77s are a direct EL34 replacement, and those might get you where you want to be. They're cleaner than EL34s, with a bigger bottom; I've heard them compared to an EL34/6L6 hybrid tonally. I haven't switched 34s out for something else, as I'm rather fond of how they sound run right on the edge of breakup (very loud!) and the only real options are cleaner tubes, which isn't what I'm looking for.
Glad to be of some help!