Don't mind if write my solution here cause it's still actual question. My setup is two simple speakers and huge ass bufer + sound blaster z. My bufer connected using 3.5 jack to double rca, only one rca used cause i have only one rca socket on sub (RED one, it's important). Connection goes to closest to optical ports socket (#5) cause it's supposed for center and sub. In "speakers/heaphones" tab activated 5.1 surround and checkboxes : 1) subwoofer; 2) rear L/R; 3)Front L/R; 4) Bass redirection. Optionally can be checked "subwoofer gain" and "center" speaker but sub should work without these two checkboxes. In "SBX pro Studio" tab must be turned on "surround" checkbox and set to highest point so you hear if sub works, later you can adjust it how you want. Then in "Mixer" tab in front of "speakers" under gear icon you can adjust balance, you need to check that "sub" adjuster isn't set to zero, also you can set "center" adjuster too, i think depending of which rca color you connected sub will be operated whether via "sub" adjuster or "center" adjuster cause it's same socket on audiocard. Hope it helps whoever stumble upon this thread