Thanks Skank. I already use a master stereo reverb bus, and have bounced stuff for a while, but to run a decent amp simulator I can only run one instance. I've tried using more than that, and have even bussed guitar tracks to the same plugin (which has its limitations), but any more than that and everything goes wahoony shaped.
My Late 2011 Macbook pro is feeling the strain of a lowly i5 processor. My software is fairly dated (Pro Tools 10.3.10) and my OS is 10.8.5, so they're not the fastest, but I can at least use AAX format plugins (instead of RTAS), which helps to reduce the load somewhat.
Maybe I just need to find an alternative to Amplitube, as it's REALLY processor intensive.