I have a pair of R528 connected with the speakon connection and LFE.
This is the best solution if you intend using the preamp or integrated AV amp in Direct mode (Pure mode for some) for music bypassing all the processing (Eq and bass management).
The problem with almost all RELs is that they are not capable of handling signals below 25 Hz. There is a steep cut off at around 23 Hz which is only good if you think that such solution can preserve your sub from being blown out by some films' sound effects (read the thread about the first few seconds of Edge of tomorrow).
When palying films they are OK, but do have some limitations in terms of the impact they are capable of providing.
Therefore, if it is flexibility you are after then REL is a very nice solution. If it is great movie experience you are looking for, then REL may not be the solution for you.
In Europe their appeal has somewhat fallen due to the steep price and not great customer support. Spare parts are scarce and often not available for products that have just been pulled off the shelf (including speaker cones and grills).
I have also discovered that the R528 suffers from a resonance at 25Hz coming from its passive woofer, which on some program may generate a flapping (ie during the first few seconds of Edge of Tomorrow and during the THX life trailer).
When I will change these subs, I am quite sure they will not be Rel.