Your Digitech processor is designed to be a preamp and an effects processor. Using the preamp in your guitar amplifier creates a conflict between the two preamps. Try connecting the main outputs on your processor to the effects return on your amp. This bypasses your amp's internal preamp and should clean up your sound.

If you want to use the preamp of your amp, connect your guitar directly into your amp. Use the effects loop of the amp to connect to your effects unit (send and return). Then turn off the preamp in the Digitech unit by editing the patch and bypassing all the preamp parameters.