Phase adjustment is frequency specific; I assume it is set at the crossover point for most subs. This allows one to marry the frequency response between speakers and subs at that point. One could do it by ear with a test tone, but then the user is just closer to having acoustic measurement equipment. I assume this complexity is beyond what REL wants to market as novice-friendly setup. REL also instructs setting both volume and crossover point as low as possible, again done by ear. I think that makes phase adjustment less meaningful. The ability to adjust polarity is more vital in my experience with both REL and JLA. Even with phase matched the sub is playing many cycles behind the speakers.
