That's probably the ADPCM. The ADPCM encoding is the biggest difference between S-YXG50 and the MU50. All the long samples, like the acoustic pianos, is always U8 on S-YXG50, but mostly ADPCM on hardware. Both formats are 8-bits per sample, but the ADPCM should come out with a lot lower noise, perhaps at the cost of some frequency response

until recently, it wasn't possible to decode the ADPCM accurately, any samples that used the running delta modes had issues. Like *very* recently, I had to rewrite my decoder yesterday based on tarboh's work

I don't have a good bar chart setup for this but here's the raw data for sample formats on the GM mode patches in GS mode across S-YXG50, MU50 and MU80
https://pastebin.com/un97i9My
The u16 samples used by S-YXG50 here I believe are mostly the same sine wave used at the very highest keysplits. MU50's equivalent sine is 12-bit

Oh. i see what's goin on now. 8 bit adpcm actually can store considerably more dynamic range than 8 bit uncompressed. so decompressing them to 16 bit wav is actually correct.

Now i'm curious what would happen if the tg300b bank and mt32 got replaced with roland samples converted to the xg AWM2 format. 😀

It would be awesome if someone could make one spftsynth to rule them all. one that plays back all xg and gs midis correctly, and uses either the GS or XG gm1 or gm2 banks depending on which of xg or gs was played back first. 😀