A point to be aware of is that most preamp and line stage designs which provide two pairs of RCA outputs drive both of them from the same output stage, and simply jumper the two connectors for each channel together inside the rear panel. And likewise for designs which provide two pairs of XLR outputs. From an electrical standpoint that is essentially the same as using a good quality splitter on a single output.
Also, in many (but not all) designs providing both RCA and XLR outputs the signal provided to the RCA connector is the same signal that is provided to one of the two signal pins on the XLR connector.
All of that applies, for example, to most Audio Research line stages and preamps.
And what that means is that the output stage of the preamp will see a load impedance equal to the parallel combination of the input impedances of the two power amps, which will be considerably less than the input impedance of either, and may be too low to be optimal for some preamps. For example, Audio Research recommends a **minimum** load impedance for most of their line stages and preamps of 20K. If say one amp has an input impedance of 30K and the other has an input impedance of 50K, their combined input impedance would be (30 x 50)/(30 + 50) = 18.75K, too low to be optimal.
FWIW, I have used the Audioquest splitters Yogiboy suggested in applications involving suitable impedances with fine results.
Finally, keep in mind that most powered subs have relatively low input impedances for their line level inputs, typically somewhere between 5K and 20K.
Regards,
-- Al
