Yep you're right there, there is no need for a 3 channel sound system so instead the subwoofer amp splits the signal so that the woofers&tweaters handle left and right and the subwoofer handles everything below a certain frequency. There was no point for any software development as you're not using any extra channels. 5.1 and 7.1 is encoded with 6 and 8 channels in mind, and the same with 6.1 but as far as I am aware, 6.1 was still 6 channels, just the amp in the subwoofer is supposed to do the job for you and split the frequencies, much the same way a tweeter and woofer does in a normal stereo system.