Acappello is upscale/expensive, no? That wouldn't be *my* choice before a concert, but that's me.
My rec is a walk up Bloomfield Ave to Next Door for everything from a fantastic burger to salads and full-blown entrees. It's BYO (as is most of Montclair). Other options:
Kei Kabob for authentic Persian (right on Bloomfield near the Wellmont)
Raymond's on Church Street (think upscale diner)
Again, ALL of these are BYO and all will be busy, so go early!
ETA: My one non-BYO rec is Egan and Sons on Walnut Street which is a longer walk but totally walkable--decent pub food, full bar
There are all sorts of restaurants within walking distance to the theater. You can check out the new "gastropub" Pig & Prince, if you want to spend a ton of money for small portions of mediocre food. Cuban Pete's is great, casual, and affordable - but will most likely be mobbed before a show, and they never do "busy" well. There are two decent sushi joints, a few italian places. Ethiopian, Peruvian, French, American.. even a standard "Office Beer Bar/Grill."
I always go to Ah Pizz, 7 N. Willow St, just a block and a half from the Wellmont. Love their Neopolitan style pizza and salads.
If you are not in the mood for Italian, you can't go wrong with Cuban Pete's 428 Bloomfield Ave, which is also about a block away from the theatre. Noise is an issue though, so not a place to go if you want to have a good conversation.