Ah yep good call. I wasn't sure if it was a quick fix. I'm worried it's something with the cable connections but when I ran wires to the 2nd socket (I did all that on a breadboard), both SIDs were detected. Though it could be firmware - I'm noticing it'll do stereo if the filter isn't involved.
I gotta say, since I have 8580's (well I have one precious 6581 from my actual C64), I forgot how deep the filter is on the 6581's. Turn it to 6581 mode was quite a treat!