The 'U' in the IC name means that it's an inverting buffer, so that might have an effect on the sound - I'm not sure. It may end up sounding the same anyway!
The diode I don't think matters - it looks like it's just there for power supply protection, so it won't change the sound.
If you want to play with the gain, try changing R3 to either 50k or 200k, see which way you need to go to make it work for you. Or try a different pot value for the gain pot. Maybe a 500k would work better for you. Experiment and see.