In the stereo tool, the phase rotation slider’s value actually never reaches full 100% or 90 degree (or was it 180?).
the residue bass seems to come from the surround parm, I replaced the surround effect with a gainer inverting only one channel of a mono sound, then collapsing to mono after it seems to cancel 100%.
I dunno, surround was never advertised as pure side side signal generation, maybe it works differently and treats the bass some special way, because, eh…? Render your setup & boost it and you will see it is the low end from whatever stuff was put through.
OopslFly, using gainer is a great idea! Damn 
I think “surround” is just phase rotation. at 100% it should do 180 degrees. That slider is very old, from the times before Renoise attempted to sound professional, I believe. Could the devs give me a statement, is it (a bit buggy?) phase rotation, or does it do more than that.
It’s not only bass content. There is also some attack part of the normal sound getting thru.
It’s negligible sound wise, but maybe it reveals something fishy going on in the signal path?