Free, Bad Company, and Cream would all be associated with "early" Marshalls. There are actually lot of subtle variations in "early" Marshalls from the Cream era (late '60s) to the Bad Company era (mid-ish '70s) -- and you can read probably more about all that online than you may need or want to.
But, anyway, those guys used various different incarnations of those amps. I presume Clapton-era Cream was a JTM-45/100 with EL 34s? Paul Kossoff used various Marshalls, including JTM-45/100s, Superleads, and Superbasses, probably variously modded, though much of his particular tone might be associated with his use of "bass" speakers (55hz, as in the then-current Marshall bass cabs, instead of 75hz in the usual Celestions intended for guitar cabs of the period). Mick Ralphs likewise used 100w Marshalls, probably Superleads (?); he also used the "Echoplex preamp" trick, I think.
So, the obvious choice of an amp model for this kind of tone in AmpliTube is the
JH Gold. This is technically part of the Hendrix package and (as I understand it) a careful model of a genuine Hendrix amp, but otherwise basically a JTM-45/100, the major difference being that it has (or is modelled as though having, anyway!) KT66 tubes in the power amp section instead of EL34s. It's one of my favorite amp models from AmpliTube.
Humbucker guitars (like LPs) are kind of "dark", and Marshalls are kind of "dark", which is why a lot of players back then used treble boosters (alas, models of any of these in AmpliTube ... yet
), but otherwise you may find backing the bass knob way down and cranking the treble (and perhaps mids and presence) is a good start. You can also "jump" the inputs on the JH Gold just as on a physical 4-input Marshall; I don't think as many players from back then did that as we sometimes now think, but some did. So, if you think it sounds good, do it.
And then, if you want to invest in the Fulltone SSTE model (IMO, well worth it), this is basically a modernized Echoplex clone, and you can use it as a preamp (the model is presented as a "rack" unit model, but you can put it in "stomp" position for this purpose) that boosts the signal, especially highs, and rolls off some lows -- which is just what various players of that era did with original Echoplex units. You may need to refer to the SSTE manual (available online) to help tweak things just how you want, but it's pretty straightforward. The SSTE model has become my favorite delay in AmpliTube. (There was an Echoplex-based model in the original Hendrix collection, but I didn't actually like it so much, and used the Echoman-based model instead -- until I got the SSTE model!
)
Then, obviously, you want everything to run into some kind of vintagey Marshally cab, though there are various stock options that could be appropriate (e.g. 4x12 Closed Vintage 1 & 2, 25C, 75C, and J120), and the choice of any one of those can actually make quite a difference to the final tone -- even more, once you start messing around with swapping various speakers in and out of the cabs. I might start with the 4x12 Closed Vintage 25 C model, set up a basic tone I liked on the JH Gold, and then start messing around with different cabs and speakers.
If you want some variety on the basic Marshall vibe, try one the HiAmp (i.e. based on HiWatt DR103) and the Orange OR120 models. A similar EQ philosophy (i.e. lows down, highs up) probably applies here. The HiAmp cab model and its modeled speakers are also fantastic. So is, in fact, the Orange PPC 412 cab model, though it is
technically a modern design with modeled Vintage 30 speakers that didn't appear until the mid '80s, I think. The modeled Slash Marshall cabs also sound great, but also have Vintage 30s. You could use these to slightly modernize your tone, or you could even try slapping more vintage-style speaker models into these hypothetically "modern" cab shell designs. I've had results I've liked from matching up the Closed 4x12 J120 cab model with one of the Slash cab models. Go nuts!