Just use the RCA preamp outputs to the stereo inputs on the sub. I have an SP1 which I will likely be selling and I use it in a 2.1 system myself. It has both pre-out as well as a sub out with crossover function plus many other set-up options not found in the stereo only preamps. For two channel sound, even with a sub or a pair of subs, set-up is just the same as with a stereo preamp. That is that the main outputs can be used as sub outputs, you just have to use the crossover on the sub to control the frequency balance in room.