I have to guess here, because I don't maintain the menu software and SID player of the Chameleon. However, I do know that every SID file also contains meta information about the author, year, copyright and the SID type that it plays best with. So what you observe may not be "defaulting" to a 6581 SID, but merely a choice of the SID file author that the files you're choosing are all composed for a 6581 SID.
Tobias can answer this when he returns from his holidays on january 6th.
Thanks for the answer. I will try to investigate this further.
Another question which is not directly related to SID. How much of the real VIC II is actually used when using the TC64s VGA output? Is it only syncing with the VIC II to get smooth scrolling? Would different types of VIC IIs in any way affect the picture quality you get from TC64s VGA output?
Also, would it be impossible to let's say have an option in the TC64s menu to use the real C64 CPU instead of the emulated one so one could enable the casette port, but at the same time keep the VGA output enabled? Sometimes I would like to load real tape games (nostalgia reasons) and at the same time have the VGA output working