I've used RELs with LFE only in the past and they worked fine.
On a more recent small REL addition to a different system I decided to try the high-level connection instead and found that I did prefer it over the LFE in that case. Technically better in some way or not (or worse), I subjectively liked it better.
My suggestion is that you try wiring it up both ways and give it a listen. It's not terribly difficult to do if you have access to the rear of the amp. If you find no audible difference and no subjective preference then I would suggest that just LFE is the easier, cleaner method.
I've been enjoying the bass in that small system enough that I recently decided to give a larger REL a shot in one of my other primary systems. That new REL arrived the other day and I've been working with it for a few days. I am connecting it in the way you are referring to by high-level + LFE as there are a set of surrounds in that system now as well.
So far the high-level connection is working well in the mixed setup. Your basic bass management for the mains will be done on the back of the REL (crossover and high-level volume, the crossover setting will not apply to the LFE input for the other parts of the system) and you can do this by ear or with a mic/level meter. You'll set the mains to "large" (or whatever equivalent that doesn't round bass to the sub from the fronts signal) and Room EQ can be applied just as though it was any other speaker to smooth out the final mains output with the sub mixed in.
I'm having a bit of an issue with the LFE side yet but working on it. In my case, the LFE level seems to be substantially lower than the high-level input signal and I'm unsure why. I can increase the LFE level but going up into the higher range, I'm finding some hum on the LFE input. It's volume/level dependent so I suspect it's just EMI on the cable (it's old). I was still able to get it all set up but I would have preferred to get a little more base output on the sub for the algorithms to work from. Still this is just applying to the surround pass-off and, in use, it sounds just fine as it currently is.
I've ordered a better double-shielded cable that should arrive next week and hopefully clean up some of that hum and allow a little more leeway on the level setting . Then I'll try tuning again. I'm not sure what's causing the signal level discrepancy (maybe just different gain levels between components as the amp and the LFE output device are separate) but I also have a note in to REL support to see if they have any comment on it.
I hope this has been helpful and relevant to what it sounds like you're considering.
