REL subwoofers have both line-level and high-level speaker inputs and they recommend using the high-level speaker input as the preferred input (or both if using with a stereo preamp and a home theater processor - most REL models have dual level controls, one for each input type).
On the other hand, not all subwoofers have as high an input impedance or as high quality electronics as the RELs and some could possibly have a small negative effect on the main speaker sound quality using the high-level speaker inputs.
Some users that have a preamp with only one set of variable pre-outs (that go to a separate power amplifier) sometimes use a Y-adapter to split the signal between the power amp and subwoofers. In that scenario because two inputs are combined in parallel, the input impedance that the preamp sees is reduced, and some might find the speaker level inputs sound better.
In your case with an integrated amplifier with preamp outputs, or for those that have a preamp with two pairs of buffered (isolated) preamp outputs, there is no additional loading on the preamp stage. One could also argue that when using line-level outputs the signal is going through less electronics or amplifier stages, i.e. a possibly purer signal.
I usually use line-level inputs when possible (I've never had an REL subwoofer), but each system is different and it's hard to say that one method is superior over the other, particularly with such a wide variable as subwoofer electronics.