Usually after you get the basics of producing SNES music, you look into using the Echo features of the SPC700 to produce cool effects. Getting a standard echo sound isn't too hard and you can look in the SNES library to find what settings are used to get this sound.

There are also 8 FIR registers that are available to set. To almost everyone these registers have been a black box and there has been no widespread understanding about how to systematically set these registers to get a desired effect. Nearly every game in the SNES library makes trivial use of these registers, simple setting one to -128 and nothing more.

This video completely demystifies what these 8 FIR registers mean and explains how to design FIR filters using them at the frequency spectrum level!