northerngames is correct, they lack 15khz rgb. I've got a 36" gateway destination that rocks, DC/GC/Naomi/Atomiswave, but thats it.
Ben, I thought you sold off all your cabs? What cab do you have left? And is it just the chassis that went bad, or did the tube break?
If you are just looking for an upgrade to VGA compatability, email excellentcom and cosmicco as they both offer chassis upgrades for Astro and I think Blast City.