on a side note how do you change the memory of the zx81 from 1k to 16k using the Windows inbuilt UI?
You can't unfortunately, only from commandline using -ramsize. I added a significant softlist for the zx81 (will be in 0.183) in tzx format, so this format has been added to zx81. Due to most zx81 titles requiring 16K I suspect most users will think they are not working since the machine doesn't actually tell you how much RAM has been assigned. I'm tempted to change the default ramsize from 1K to 16K.
Which machines support a .cass format? I can't find any mention of this format in the src. All tape machines support wav as a minimum and any other formats specific to each machine are added.