Roger, the S900 has a 5-band Master EQ, whereas the S975 has an 8-band Master EQ. This allows more refining of sound, including guitars (I think boosting band 6 slightly improves guitars).
Also, the S975 speakers deliver a MUCH bigger, dynamic sound than the S900. I think the S975 makes the voices I port over from my S900/S910 USB setups sound better.
- Royce