Who makes the best subwoofer for music? REL or JL AUDIO? REL uses High level, JL AUDIO low level with EQ. Which will be better for music. 
People always choose only the most popular brand of subs which are not really the best
People always choose only the most popular brand of subs which are not really the best

I'm wondering why they are not really the best, Is it due to one of these; better price, better advertising, better distribution, or simply a better product.

One could ask why are they rejected. I hope it's not exclusivity and bragging rights?

There must be a reason I'm not understanding. Is anyone who purchases a popular model judged as inferior?
I have owned a pair of S510 and now a pair of fathom113v2. I had no problems with the sound or build quality of the RELs. They have the benefit of speaker level inputs for those of us without two line outs. They are easy to lift using their handles. With the help of an a remote acoustician, I was was able to well integrate and flatten the frequency response up to 60Hz where there was a 10db node that could not be solved because it was vertical/axial coming from the main speakers.

The acoustician recommended a JL CR1 crossover, so I bought the subs along with it. I had immediate buyers remorse because they are really expensive for me. The package arrived a few weeks ago, and I have just used factory recommended settings. The fathoms are much more feature rich than the S series, including auto turn-on, auto-equalization, a low frequency filter centered about 20Hz as well as a phase knob. They come with covers that I also use. I have only set it at the recommended 80Hz, set the 20Hz filter to -6db or so zeroed everything else. The CR1 eliminated the 60Hz issue. Nothing is particularly optimized, but bass is tighter with better resolution. The bass integration with the horn section of my JBLs is now better than with JBLs without subs, which has been an issue since I first bought the speakers. I would be happy with either brand, it is just that the JL offers a relative abundance of adjustability and I will not have to consider subs again.