What you’re doing isn’t clear to me, but it looks like you’re combining the left and right channels together at the amplifier output to create a mono signal for your sub. Is that correct? If so, the problem I see is that most amplifier and preamplifier outputs are wired in parallel. So if you tie the L and R outputs together at an amp output, you’re likely to have mono signals at the amp's other outputs, too.
Connecting dual subwoofers to a stereo preamp
I have a 2 channel Adcom preamp. There's no subwoofer output (LFE or otherwise). For many years I've run my single M&K subwoofer with a double RCA cable, left to left and right to right from the tape outputs on the preamp. My 25 year old subwoofer is failing. I'm probably going to get a pair of SVS SB 3000 subwoofers. I've read quite a bit of opinion on whether or not to run dual subwoofers in stereo. I'd like to try it just to hear how it is then I'll decide. The SB 3000 has line in left/right RCA connections. If I don't set them up in stereo I know how I'll connect them. Not a big deal. But if I decide to hook them up in stereo then I'm not sure. I think I know. There's really only one way. I'd run the left output of the preamp to a Y and then run the Y to the left/right of the left subwoofer. Then I'd run the right output of the preamp to a Y and then run the Y to the left/right of the right subwoofer. I'm not asking about whether or not stereo subwoofers make any sense. I know the arguments here. I guess my question is, is there any actual problem with Y'ing the single channel of the output from the preamp to both the left and right on a single subwoofer? Should I even bother with Y'ing them? Maybe it makes more sense just to run to the left RCA on each subwoofer and leave the right subwoofer RCA connector open.
Thank you in advance and please excuse my terrible drawing! 