Effects of using y-cable from pre outs

I have built in single channel sub cable. Older receiver only had 1 single sub output. I'm not about to tear into walls or add more cables. If I upgrade to an integrated amp with stereo pre outs...what effect will using a y cable to connect stereo outs have? Phasing effects? High noise?
The y cable will connect both channels together so that the signal going to the sub, left and right channels will be the same, effectively mono.

Either get an amp with dedicated sub outputs, which should be electrically isolated from the main pre outs (if the receiver has them) and use the y cable, else you will need a mixer to blend the left and right signals to give you a mono feed to the sub.
I don't know about LFE channel in theater applications, but for 2ch, everything below 80hz is mono, anyway.
As Ghostrider indicates, if the pre outs are part of the main signal path (i.e., if they are jumpered to the inputs of the power amp section of the integrated amp), the result would be a mono signal going to BOTH the main speakers and the sub.

If the pre-outs are separately buffered and are not in the main signal path through the integrated amp, using a y-cable will PROBABLY work ok. But I would first check with the manufacturer of the amp, to make sure that connecting the two outputs together will not cause damage or compromise long-term reliability. It's usually ok to do so, but conceivably might not be depending on the design of the buffer stage.

A mixer, with high input impedance and low output impedance, would be the most ideal solution.

-- Al