Has Rel fallen out of favor with audiophiles?

"REL's top subs" means Studio III, Stentor III, or Stadium III and the current Reference series.

Dave 

Considering the almost infinite differences in systems, rooms, and personal taste, adding any subwoofer/s is quite a subjective subject.

Subjectively, I find the performance needs for my primarily analog music only system much greater than for my home theater. Home theater controllers 80Hz standardization makes the need for an on board digitally EQd subwoofer unnecessary. 

In an in home music only comparison with two other subwoofer owners and their subs the only noticeable difference between digitizing the extra low frequency signal was the ease and preference of system and room integration and their stunningly superior performance. The narrowing sonic differences between analog vs digital doesn't seem to be audibly present at this region of the frequency band especially when considering their equalization and optimization control advantages.

To answer the original poster question, yes it is about the quality of integration. In our comparison the REL Studio III was the quickest to set up as per the manufactures instructions. Unfortunately, it was unanimously the poorest performer in all other aspects. To be fair the most expensive and finely crafted British made Studio III was the oldest sub in the comparison and I haven't kept up with any of the brands current advancements. The other two digitally controlled subwoofers in the comparison are currently using substantially improved second generations of control.

The ultra low Hz is simply icing on the cake.   

I have no thoughts about REL, so have not heard them. And, I have read excellent reviews about them.

To deal with a specific issue of lost bass energy due to a large room with open front wall corners, I purchased a pair of semi - DIY GR Research/Rythmik open baffle subs. More like a kit than complete DIY. With help from others here, I would find out that the way these subs load a room, would work best for my situation.

it's also about your taste. I have been working towards a very relaxed sound that is also wide open, detailed, and natural. My system is analog, but many who have built these have digital front ends. With the addition of these subs not only did the combo maintain my taste in sound, but my system plays louder and the soundstage is big and full.

As for the bass, it's completely natural and added focus. In combination with my Horning speakers there are zero coherency issues, so the combo works seamlessly. The subs are capable of dropping down to 14Hz, so I have wound up with a truly full range system that is also completely relaxed. I never thought I could get this without spending what I absolutely cannot afford on ultra hi end speakers.

With that said, this may not be the best option for you. You need to look at the physics of your room before deciding on the type of subs to use. My sub project was a leap of faith, because you cannot listen to them since they are DIY. You may find them at a Show, but that may be rare. IMO- The most important part is matching subs to your system and space. Or, just get better speakers to handle the job.
Kenny