I haven't read the manual in a while, but I think it has a good section on this. Basically, though, you would pick one setting and leave it there. The switch only affects the 7.1 analog input. "Bypass" provides no bass management to that input. "HPF/LPF" has an analog 80Hz crossover network that redirects bass from the seven full-range channels to the subwoofer (mixing them with the LFE channel). "Digital" converts the 7.1 input from analog to digital, steers it through the 970's DSP section for the same bass management that digital inputs get, and converts it back to analog.