Like everything in life, it comes down to trade offs & compromise. Yes, 20hz has been the mark we have been taught to shoot at. But in reality, very little music goes down to 20hz. Then too, there is the trade off of size. In order to get down lower, you need a bigger box. Plus it costs more. Then there are the specs. Is it rated conservatively or is it rated better by manipulation for sales.
The nice thing about the Rel is how easily it blends and how much bigger it makes your main speakers sound. Plus it uses high ;level input from the speaker terminals on the amp. I have had them for 20 yrs now and was one of my best upgrades.