Only subs with DSP processing. In fact it applies to any DSP device you insert in the signal path. In Randy's case for example he also has a SMS digital equalizer in the subs signal path so he is delaying 6 ft, 3ft for the 600's DSP and 3ft for the SMS DSP. For subs without DSP use normal distance then experiment up and down a foot or two at a time and listen if the bass tightens up.
The distance setting will also depend on how much the sub is in phase with the main speakers. If you have the sub well calibrated to the mains, it should not sound much different from the normal distance (after adjustment for processing). If you find that the sub tightens up with a big variation from the normal distance, it is likely that you need to work on getting the sub more in phase with the mains.
