How does a REL subwoofer add dimensionality and depth to a sound system?

If you have multiple well placed subs, usually it takes your what is likely pretty uneven bass response from the mains and evens it out. Limiting woofer excursion on the mains can also significant reduce their distortion.

Unless you are using a sub array, the settings on the back of subwoofer, even a basic EQ function, is in my mind, while not worthless, not that far off.

IF you are going to go the sub route, please invest in a microphone and learning something like REW. For the most part, you are wasting your money if you don't.

If you are an analog "purist" let it go. Sub bass management is best done with DSP. Your hearing acuity is low, and speaker distortion high at bass frequencies. Nothing an ADC / DSP adds to the system is going to impact the SQ, other than in a positive way.

If you go the way of @oldhvymec , treating the room significantly, then there is less need for EQ/DSP, but it can still get you that last bit of tuning. I have both. DSP gives you more flexibility with placement.

I expect for most audiophiles, Paradigm subs with built in room correction would end up being superior solutions to most competitors. Even without it they would be a solid choice.