Hi Tobias,
Yes, reconfiguring seems to do something. After the first install: the 8580 was recognised, the 6581 was an "unknown" SID type. After contact with the SIDFX people I flashed a new SIDFX firmware that allowed me to overrule the auto detect so I could manually set the SID type. That worked. I hadn't looked at CH2's SID emulation settings. ( I didn't realise that the code actually runs inside CH2 and that this code actually scans simulated SID's).
I set the emulated SID's at stereo on $d4xx and the real stereo SID on $D420 like you suggested, I configured SIDFX to have the second SID at $d420 and I got sound again. Tweaking with SIDFX configuration tool does something because there are configuration settings that completely mute both SIDS.
There is another issue, that also troubles this water: SIDFX has two connectors: 1 that reads the profile (you can have three profiles which allow for three different SID settings: for example left profile is Mono SID1+SID2, Center is Stereo SID1 and SID2 at $D420 and the right profile for example Stereo SID, with SID2 for example at $D500). The switch used for selecting is a 3 way switch. A second 3 way switch (that works because I swapped them) is used for selecting how audio channels are mixed. That switch should have 3 setting left/center/right but it only has RR- where the - (minus) is an undefined position that should technically not be possible. (switch is both in left and right position) so I can't determine how sound is actually mixed. It mostly sounds, like I hear only one SID. I do not know how the software running inside CH2 reads the status of the switches.
So now I am looking for a solution that does not emulate SID but still gives me the possibility to load software from an SD card. The only way to test and verify the SIDFX is by completely removing the CH2 from the equation.