Buy a small(er) mixer with 8 channels for you drums, mix each drum how you want and take the stereo output so its only one channel.

As for the device I would suggest the Delta 1010 (300-400 used, 500 new i think) its a pci and rackmount that uses one cable to transfer everything from the breakout box. A cheaper suggestion is the Delta 1010LT, pci card with 10 channels worth or inputs. 200 new.

Phil