I think the effects loop in the M13 provides an assignable slot. So if you are only going to use external dirt pedals then you can place them post-compressors. Di din ako sigurado ha. M13 users, feel free to chime in.
You can assign multiple delays to multiple slots. I got an M9 thinking it like having 3 DL4s in the space of one.
While on the topic, instead of of using the Tri-Chorus model, try chaining 3 different choruses together. Sweet.