Hi there! I saw a similar question answered a couple of years ago, but my components are a bit different (hence the new thread). I'm trying to properly include a Polk Audio PSW10 subwoofer in my setup.
Currently, I have an Audioengine D1 connected to my PC via USB for power as well as an optical cable. Connected to this DAC are my bookshelf speakers (a pair of M-Audio AV40s)--these are connected using a 2-Male to 2-Male RCA Audio Cable. Now, the only way I've managed to make the subwoofer work is by using a 3.5mm Stereo to 2 RCA Cable Male to Male cable and simply plugging it into the back of my motherboard's 6-channel audio (the green speaker port specifically), and then enabling the Stereo Mix option in Windows 10's Sound settings (under the Recording tab). The motherboard is an Asrock Z370 Taichi. However, I noticed that the bass is well off rhythm this way, most likely because it's taking signals for both the left and right speakers all at once. I did try plugging the subwoofer into the sub/center port (the orange one) in my motherboard, but was unable to get any response from the PSW10 despite trying different settings in both the Stereo Mix and the Realtek HD Audio Manager. I'm a novice at this so I'd appreciate some help.
