The biggest decision factor here is their very different modes of connection/setup and how they relate to your particular system configuration. REL is pretty much commited to high-level (speaker-level) connections is contradistinction to almost everybody else in the business although they have made gestures in the direction of low-level (line-level) input. JL offers both and definitely leans in the direction of line-level which definitely permits the implementation of proper system integration, crossover and equalization. IMHO, those facilities are necessary for successful use of subs and, without them, integration is a crap-shoot.
Also, REL's old canard about high level sub connections being valuable to preserve the system/amp tonality is part of that crap-shoot. Whatever tonality we perceive of a sub's contributions is determined by the harmonics which are still provided by the main speaker. What if the reason you are thinking about adding a sub is really due to an LF weakness in the very amp that is now running mains and sub?
If wanted, anyone can use a standard subwoofer in the high level mode - just build a resistive divider with a ratio around 20 - 1000 and 47 ohm will be perfect. However as you say, why doing it? Most of the time all it will do is limiting the bandwidth of the subwoofer to the bandwidth of the existing power amplifier - a complete disaster with most tube amplifiers that have limited bass and high distortion in the low bass. Also, if you use a single subwoofer in a few cases this technique will also introduce noise in the subwoofer.
Just a technical advice, use whatever you find it sounds better in your system.