.....Buck, from your very good details I surmise this....the two input options at sub level are making it possible, one last time, for pure left and right to be inputted into the sub where it will be mixed mono by the pre-amp section of the sub....now, your mixing of left and right already happens at pre-amp level of your receiver to output the signal to the sub....KILL the Y to the sub....that's just more splitting by rca's to bring possible noise/interference/distortion....AND, this is assuming that the woofer element of the sub is NOT a dual voice-coiled woofer....anyhow, if your system were mine, Buck, I would turn the sub off and listen to the other 7 speakers....I would choose ALL the speakers less the sub to be controlled by the small-to-large adjustment....I would bring the adjustment as LARGE as possible NOT hearing bass bottom-out by distortion or confusion to the ears....guys, if you're not after CLEAN with your system, you're NOT an audiophile waiting for the right equipment to show up that you want to be pushed, ""FLAT""....after adjusting the small-to-large adjustment on the regiment of mains/center/surrounds/rears to CLEAN, bass-wise, I would then turn the sub on, and bring the volume of it to supporting the cause in the upper room, CLEANLY....I would NOT want the sub controlled by the small-to large adjustment to allow the "option" of the frequency cut "ceiling" of the sub's signal to enter into the upper room a tad, and the word emphasized here from start to finish, is CLEAN......