It's all good mate, I think there's a few factors working together which explain why Marshall amps are so loud, there's the directional beaming of the sound caused by the sealed back stopping the sound escaping, so it can only propagate from the front, there's also the harmonics generated by the Marshall amp when it gets pushed into overdrive, the interesting thing is that the human ear perceives the extra harmonics generated by the amp as a kind of loudness, what's even more interesting is that brass instruments, like the trumpet, Saxophone, Tuba, Trombone, etc, tend to behave in a similar way too, and as it just so happens, the human ear is very sensitive to midrange frequencies, and the human ear perceives midrange frequency harmonics as a feeling of loudness too, I think perhaps when Jim Marshall was working with Ken Bran and Dudley Craven to create what would become the first Marshall amps (the JTM 45s) he might have purposefully chosen to go with a closed back speaker design in order to create the sound that guitarists were describing to him, they wanted an amp that would be loud enough to be heard along with the drummer etc, the JTM45 was actually based on the Tweed Fender Bassman, which was originally intended as a bass amp for the Fender P-Bass, but, guitarists found that it made a great guitar amp.
And you are right too, I have seen a Hartke Bass amp where the speaker box actually did have Bass ports as well as a sealed back.