We stopped moving for a few hours, so I headed down into the studio to make yet another assault on The Grim Roper mixes. The mixes with vocal and finger-picked guitar are generally sounding ok when we take them out for the truck test, but the mixes with strummed guitar sound pretty congested, which I think is because the mono vocal and the mono guitar are both sitting in the center of the mix and getting compressed together, which has the effect of causing the vocal to vanish back into the guitar.
I tried a number of things today (some less successfully than others). I think that the winning strategy may be to drop the guitar onto an FX channel and route it through a stereo enhancer, which will tend to spread out the image of the guitar and make room for the vocal in the middle of the mix. At least this approach is sounding ok in the studio.
We'll see what it does on a lesser set of speakers soon. :)