.spc files through game emu player, and probably snesapu as well though I can't confirm because game emu player prevents snesapu from working afaict, use less than half of the available range and are also very, very, very quiet, universally. Some other formats also end up being quieter than is preferable, and similarly, never use the full range, and therefore could be boosted without clipping.
Shouldn't there be a way to apply gain offset by file format somewhere? Is there a way to define this in game emu player somehow?