Your post is hard to understand.
What do you mean by fx signal? Pre-amp outputs are generally full range and always active. Don't know if that helps.
With only an avr external dsp is usually only valid on a pre-out interface with an external amp, like perhaps the sub pre-out to an sub amp, but without external amps applying dsp to other channels isn't going to be possible unless perhaps using a specific source via the analog inputs to the avr.