I wouldn't consider a Expression for more headroom over the JMP. 

At best it may offer as much headroom as a JMP, but I think it offers less. Most of the ones I've tried are running pretty dirty by 10 o'clock on the volume dial. 

What an Expresison gives you over the JMP is a much tighter sound with more gain available; less thick, more detailed, more harmonic rich; and it's a lot more touch sensitive, you can clean up or dirty down the sound with how much you lay off or dig in when picking, as well as control it with the guitar's volume.  T'wrecks in general aren't considered real pedal friendly, this is an amp that really responds to and interacts with the guitar and player, I doubt it'll like a midrange heavy pedal like a Tubescreamer, perhaps a more transparent boost would work.

To be painfully honest, it's not my tone, I'm much more of a Marchall/Tweed guy when it comes to distorted tone, I like soft, big, warm power amp overdrive, so amps like the Expression or something D-amp-esque (or even Vox and Matchless) are just too tight for my tastes (I find them raspy).  But, obviously, these amps are all legends with huge followings, and I do understand there strong points, but they just aren't my tone.  Make sure to check out the Youtube clips of various T'Wreck clones and ask around ampgarage.com.

When was the last time you had the filter caps replaced in the JMP?  If they are original (35 years old), new caps should add some headroom, tighten up the bass, and just more more live into the amp overall.  Most people don't have trouble with headroom with a 50-watt Marshall (though it depends on the gig).

New tubes and new filter caps should make a noticeable difference in headroom.  Will it be enough?  There is always the 100 watt, or maybe a Major?