Playback of old rns / xrns sounds awful on new Renoise?

I would suggest not just basing it on an old render, but actually getting hold of an old copy of Renoise.

Because, then you could be certain what settings are working, and compare those against the recent version.

http://renoise.com/archived-releases

There is also some good advice being offered in this related thread:

https://forum.renoise.com/t/was-working-on-a-track-in-2-5-now-playback-in-3-0-1-causes-issues/43579

Btw: the song sounds really good (the old one, that is) :walkman: