I wonder
why they downgraded the SID chip. Cost reasons?
At the time they probably thought they were fixing it, since the voltage
leak widely exploited for primitive D-A conversion ($d418) was
technically a bug and is the most (in)famous thing fixed between the 6581
and the 8580.
The filter cutoff frequency varied enormously between 6581 chips, they
fixed that completely in the 8580. While also changing its character a lot.
I wouldn't say it's downgraded, it's good to have both versions.