I wanted to combine the left and right preamp outputs for the sub downstream of the splitters, in order to only have to run one cable to the sub. But am wondering what that will do to sound quality of stereo signals from splitters upstream run into the inputs for the amplifier section of integrated. not smart enough to figure this out but intuition tells me this is a bad idea. please advise.
Your intuition is correct. For starters, you won't have stereo signals going into the amplifier section of the integrated, since L & R will be shorted together. Beyond that, IMO it would not be good practice to connect L & R preamp outputs together, for the sub, even if they were separately buffered and completely independent of the main signal path (although I realize some people do that, with reasonable results).
If L & R are shorted together, and the recording has an instrument that is off to one side, so that its sound is pretty much in just one channel, then the load impedance that will be seen by the circuit driving that channel will be the output impedance of the other channel.
You are probably familiar with the 10x rule of thumb for matching preamp output impedance to power amp input impedance. In this situation the ratio of load impedance to output impedance would be only 1x. To the extent that those impedances vary differently as a function of frequency (e.g., due to component tolerances), frequency response flatness would be messed up. If the preamp outputs are ac-coupled, meaning that a capacitor is in series with the outputs, bass response would undoubtedly be affected. In some cases the output driver circuit may simply not be able to handle the heavy load, without audible signs of stress. Finally, and perhaps most significantly, the volume with which each instrument is reproduced will vary by up to 6db depending on the degree to which its position is off center.
Best regards,
-- Al
