I've played 8-string classical guitar for close to forty years (mine was built by John Gilbert*), using it for my own compositions and for lute music. Since the 7th string of lutes was often tuned to a low B, and the low B continues standard fingering (including barring) into the low notes, that seemed natural. I dropped it to a low A for some pieces in that key as well, for great effect. And since you've got electrics here, and are not finger-picking, I'd think that a low B would go well with standard bar chords there as well.
So what about the 8th string, then? Anything lower than the low B I found overbalanced the guitar on the bass side (even though the Gilbert has incredible trebles). But after seeing the Brazilian guitarist Egberto Gismonti play an 8th string with the 8th tuned to a treble note (getting a "fine-string banjo" effect, with a high note under the thumb) I settled on tuning the 8th to a C just above the B of the second string. This gave me some incredible effects in the key of C when I dropped the 6th string to C as well, so I got some good music out of that. The only hitch was that I couldn't be sloppy with my thumb on the low strings - if I missed the 7th string and got the 8th, I got a wrong note way out of register.
All this is more or less in the past, since I play mostly flamenco now (my luthier just built a 7-string flamenco guitar, however - hmmm...). As for Gismonti, he went on to a ten-string guitar, oh well.
Have fun, and good luck with your experiments!
(Here's John Gilbert, who built a number of 8-string classicals - http://www.guitarfoundation.org/blogpost/754189/139600/Remembering-John-Gilbert)