The crossover, or low-pass filter in this case, will depend on the individual size and shape of the room, placement of the sub in that room, and even personal preference, which is why we don't have any recommended settings when using our S8 sub - or any subwoofer. We usually recommend starting with the Crossover knob turned all the way counter-clockwise. Then set the S6 Volume close to where you think it should be and make small adjustments to the crossover and see how everything sounds. Finally when making adjustments, assess them from the same spot each time, preferably from where you'll listen from most often.
The A5+ and our other powered speakers have fixed passive crossovers designed specifically for them; this simply means that the A5+, for example, is designed to play what they already do so nothing will change there. Adding a subwoofer, such as the S8, will pick up where the A5+ leaves off, extending the low end response.