I've got the head (Going through a Marshall 1960A) , but I've also tried the combo.
In fact, I plugged the combo JSX into the same Marshall cab I run the head through, and they both sounded identical.
The amp is obviously the same in both, but the major tonal difference stems from the form factor and speakers of the combo, as opposed to the differences in a 4x12, closed back cabinet.
I can't say enough good things about my Axis through a JSX!
I would try to play a Rivera if you get the chance before buying either a Mesa or current production Marshall. They're my favorite of the mass produced, pcb amps, though each of the three companies have their strengths. Speakers play a huge part as well.