Precisely whey the delay is for playback hardware only. If you don’t have the surround speakers, you don’t want the front sounds to have an echo. The 15-35 millisecond delay is for the hardware to handle. The surround channels shouldn’t have any delay built in unless you are going for an exaggerated delay effect. That has the added benefit of any crosstalk getting absorbed back into the front sound and not being noticeable. The only time the delay comes into effect is when the surround channels are sent to the speakers. The delay needs to be customizable to fit the playback space.