In answer to the original question, a sub can make the main speaker job easier if it is crossed over with high and low pass filters (full crossover). This way the mains are not receiving bass frequencies and can deliver cleaner upper bass and midrange, depending on the main speaker configuration (2way, 3way)..
But the issue of phase and delay has been mentioned. Phase is not really proper time alignment, it is really just putting a late signal in phase, in fact usually making it even later.
Also be aware every sub probably has a few ms latency due to the electronics, cabinet design and impulse response of a large driver). I gather manufacturers are not eager to share this info. To compensate, it is necessary to delay the mains, not the sub. I have done this. One of my subs has 12ms latency, but it is still very nimble responsiveness (it is not sluggish when time aligned correctly)
In my system, there was quite noticeable improvement from even a 6ms adjustment, 12ms is obvious to me and frankly messes with the music timing let alone the acoustic interactions/cancellations.
With all this in mind, the approach promoted by REL to use high level inputs seems fundamentally flawed. Be wary of the marketing the REL dealers use.