[albee1952]

I also think your rig should be able to track at the 64 buffer(hopefully, some tweaks will help). As a work-around for vocals, one way to lighten the load on the system would be to bounce the music down(import after bounce), then make all the tracks inactive(except for the "2-mix", vocals and vocal reverb). With a thunderbolt interface, a 128 buffer should still be fine(can't comment on 256) as it was on my HDN setup.