Sub woofer cable help please...

Thank you Al and all the rest who have posted. I appreciate the help as it turns out there was more to this than I anticipated.

Correction: In my post above, I wrote the formula to compute Combined Impedance is:

L1 x L2/L1 + L2 + CI (Combined Impedance). I should have used an equal sign in the formula, sorry. As corrected:

(L1 x L2)/(L1 + L2) = CI (Combined Impedance)

Thanks Bifwynne for the update!

I just received this advice from Kris T SMC Audio.

"What you need is a summing cable that takes the two outputs and combines them into a single cable. (2 x RCA male to 1 x RCA female.  A summing cable uses resistors to isolate the two outputs.  A regular y connector should not be used."
Does anyone know where I can buy one?
Thanks

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