I was stumbling through HeadWize‘s project archive the other day when I came across Steve Connor’s Poor Man’s Surround Headphones. He used a pair of Sony MDR-CD60 for the base and added three pairs of ear buds. One pair is used for the front channel and two for the rears. The rears have their own amp with the bass turned up to increase the separation. He used a Sound Blaster Live! card for the source. If you are interested in amplifiers and other sound projects, you should definitely have a look at HeadWize.