Does anyone know how the filters affect performance? E.g. If you add a high pass filter above 80Hz to see all low end to a sub etc.
Obviously lowpass/highpass filters will impact the frequency and phase response, depending on the filter implementation. That is what they are for.
Or do you mean how do the digital filters affect noise/distortion etc?
Since those are done in the digital domain, then if they are implemented correctly they'll have little to no influence. The bulk of noise and distortion occur in the digital to analogue conversion (or the other way around for the analogue inputs).
Obviously if the DSP is programmed to add noise and distortion then... well... you get what you ask for.