Is the signal from the Aux In also sent to the DI? (Is it summed with the bass signal sent to front of house?)
No, the signal fed in the Aux input is not sent to the DI output. It is sent to the cab connected to the amp (if no headphone is connected) and to the headphone output. If you connect headphones, the output in the cab is muted.
The DI output is fed with the bass signal only, with a pre or post EQ option.