Sub woofer cable help please...

Good comments above.

I would add, though, that IMO even if the two sets of preamp outputs are separately buffered it would still not be good practice to sum the two channels together for the sub with a y-adapter. In saying so, I recognize that many people do exactly that, with reasonable results.

To the extent that the signals on the two channels differ, the output stages that drive the two channels will fight each other, conceivably with adverse sonic effects. That would be particularly likely to occur if an instrument producing deep bass tones were off to one side (i.e., essentially in just one channel).

A more technical way of putting it is that the output stage of each channel will have to drive the stereo component of the signal into a load impedance equal to (actually, slightly less than) the output impedance of the stage driving the other channel, which will be very low compared to the impedances it is designed to drive.

Regards,
-- Al

I am not aware of any off-the-shelf resistor y-adapters. Creating one yourself probably wouldn't be too difficult.

However, a lot more information, and some analysis, would be necessary to choose a suitable resistor value (even if off-the-shelf resistor y-adapters were available). The value would be dependent on whether or not the two sets of preamp outputs are separately buffered, what their nominal output impedance is, how that output impedance varies with frequency, the input impedance of the sub, and (if the preamp outputs are not separately buffered) the input impedance of the main power amp.

The X-30 that was suggested, or something comparable, sounds to me like the way to go.

Regards,
-- Al