Love this! I've been working on and running https://sfc.fm for about 7 years now so very similar interests!
One thing I did notice was the in the SPC emulation, particularly in Super Mario World `12a Athletic.spc` there is a lot of echo / reverb that isn't present in the actual file or game, is there secondary effect processing happening somewhere, I couldn't seem to turn it off, you can play it here https://sfc.fm/sfc/super-mario-world track 16
One thing I did notice was the in the SPC emulation, particularly in Super Mario World `12a Athletic.spc` there is a lot of echo / reverb that isn't present in the actual file or game, is there secondary effect processing happening somewhere, I couldn't seem to turn it off, you can play it here https://sfc.fm/sfc/super-mario-world track 16