It might depend on your sound card. If you use the ASIO output plugin, you can use the "Shift output channels" option. Unfortunately fb2k wont let you run two instances, but you could, for example run fb2k and winamp (not the best solution I know, but if you're just running a playlist). With this setup, you would set fb2k to use the rear channels, and have winamp output sound normally (I don't know if you can use ASIO in two apps simultaneously, but its not necessary because winamp's directsound output would go to the front channels.)
Edit: I'd recommend adding a Resampler to your DSP chain when using ASIO, even if your soundcard supports the sample rate of your music (mine doesn't), if a file of different sample rate happens to be in your playlist playback will cease with a loud error beep