The stock speakers which are celestions, and I hear are very good... ?

I have a Line 6/Bogner Valve and an Esp Viper Baritone guitar tuned to drop A (AEADGB).

I know that's more than most amps can handle but I was told it should be able to handle G tuning. It works fine before and while using pedals, except for when I specifically bottom out the low end of my grunge pedal. I can bottom it out when on low volume however when to turned up for practicing with my drummer, the speakers won't play noise or will start to crackle.

I have also played a Washburn X series 7 string through it which was tuned GDADGBE and it sounded, on its own lower, but I can't stand playing that guitar.

Anyway, all I'm trying to do is get a better low end response, here is the rest of the gear:

Line 6 mkII Shortboard (connects into back via phone wire)

From amp to guitar;
Digitech BP80 (bass modeler)*
BOSS CH-1 (super chorus)
DOD FX69B (Grunge)*

On the digitech I have an amp model selected and have the EQ set to 9B 5M 7T which incredibly alters the bass especially when I raise the bass level on the DOD.

Removing all effects pedals, is there something I can do specifically with the amp itself for a better low end?