suming left and right for mono input

I have searched. Can I use a y cable to sum both left and right line level signals to feed  the mono input of my sub amp? I want to use the non-fader output of my Eclipse CD5441 (in LP mode), thru a passive pre-amp (volume control) and then sum the signals to feed that mono input.

I did some searching around the web and came up with this bit of info. When attempting to sum line level outputs there is a tendency for voltage to be backfrd into one or the other input when signals are out of phase. The inputs are not designed for this and circuitry could easily be damaged. Even if this were not so, there is a great chance of inducing distortion into the amplifier being fed due to the phase inversion and cancellation. The answer is placing a 10k resistor in series in the output path of each channel prior to summing them. This loads the outputs and prevents backfeeding. One more thing came to mind. If I install a 100k log potentiometer from the summed signal path to ground (shields) I now have a viable level control. This is needed to control subwoofer volume when using a non-fader output of a HU.  Just tie the signal path to the wiper. Tie only one end of the potentiometer to the shields. That leg is determined by measuring for 0 resistance between the wiper and leg when the knob is rotated fully counterclockwise. The other leg is left to float.