What low level input does your sub have? Many Rel models only have a single channel low level input (often called LFE) which is not intended for connection to a 2 channel stereo amp. It’s intended to take the .1 bass signal from an AV amp. Better to use the high level input as per Rel’s recommendation if this is the case.
If, on the other hand, your sub has a 2 channel low level input, it’s fine to use that with a regular RCA cable, possibly a ‘slugged’ one if it’s very long.
At this point you might want to think about the location of your sub relative to the main speakers and amp. If the sub is close to the speakers, it may be much easier to run high level cables from the main speaker input terminals using a split cable as discussed above. If the sub is closer to the amp, you can choose between the low level connection from the pre if your sub can take it, or the high level signal from the power amp output.
Xanthe’s method of using DSP to tweak the sub signal to suit your room is, I think, a good idea if you don’t mind another box and a bit of fiddling around! It would also get around the problem of having a 2 channel signal from the preamp going to a 1 channel input on the sub, as I believe the miniDSP will sum the two signals into a single channel.
