Okay, at least one SID is working fine. I believe the other is running very quietly, and not getting any Note Off signals. That is, if I press a C3 note using the default Lead patch, I get one loud signal until I let go of the key. Then I get a much quieter signal with infinite sustain - when I press another note, it triggers okay, but the same behaviour follows.
I've done a bit of a search, but nobody else has reported the same thing. The default patch bank has failed to load in properly, so I'll try to upload it via MIDIox instead and see if that makes any difference.
c-