I don't have any experience with the REL, but I DID recently own a HSU TN-1225HO with the HSU 250 watt amp. I used it with a pair of Platinum Audio Solos. Not dissimilar to the Tylers inasmuch as the bass output was already quite good compared to other monitors - I just wanted that final half-octave as well. In addition, the room that I was using them in was quite large, so it was nice to augment the bass response and save a little amplifier power for my inefficient (84db) Platinums. The result was outstanding, fast, musical, articulate, deep, POWERFUL, bass that was quite easy to integrate. Best of all, from the wife acceptance perspective, is that the tube is easy to place (in my case, hidden behind an indoor water fountain completely hidden from view in the corner of the living room). The small footprint makes it very easy to hide. In addition, it can be laid on its' side and placed behind a couch. Try that with a big "box" subwoofer.
The only product that I can compare it to in my own "real-world" environment is the Velodyne ULD 15 which I borrowed from my father after I bought the HSU for a benchmark comparison. To my ears, the HSU is just as musical as the Velodyne. The only drawback is that the HSU only goes to 25hz versus 20hz for the Velodyne. But I don't listen to a lot of organ music, so it wasn't a big issue.
Hope this helps. Happy listening!