Why do I not want to sum the left and right? Isn't that effectively what the REL does when connected through its high-level input wired to the left and right speaker terminals?
DRubin -- The high-level input does sum to mono, but it does so via active circuitry which presents a very high impedance (i.e., a negligible load) to the left and right channel inputs. But summing to mono by putting a y-adapter on low-level signals, which simply shorts them together, is generally not good practice, and may or may not work depending on the circumstances.
First, it will not work if those same inputs are fed to the main active speakers, or to a power amp driving the main left and right speakers, because it will cause a mono signal to be fed to those speakers. Also, the same thing will happen if the preamp provides two sets of outputs which are simply jumpered together internally (i.e., not separately buffered), and you feed one set of output jacks into the y-adapter and the other set of output jacks to the power amp driving the main speakers.
Those situations are not applicable in this case, because you are proposing to use the balanced outputs to the active speakers, and the presumably separately buffered unbalanced outputs for the sub. However, it is still not good practice, and may not work with some preamps, because since you would be shorting together the left and right unbalanced outputs, you will have their driver circuits in the preamp fighting each other. Depending on the preamp output impedance and drive capability, that may or may not work, and conceivably could even cause damage.
Is your REL one of the models that has two line-level inputs, one with 12 db gain and one with 0 db gain? If so, the best approach would be to use a 12 db attenuator in series with one channel, into the input that has 12 db gain, and the other channel directly into the input that has 0 db gain.
If your REL has only a single low-level input, then Bob's suggestions sound well worth looking into. Or else let us know what preamp you are using, and perhaps one of us or someone else can determine whether shorting its two unbalanced output channels seems advisable.
Regards,
-- Al
