Dumping an arbitrary tape while maintaining the block structure, is quite tricky. Each tape block can be an arbitrary size, with an arbitrary number of filemarks in between runs of data blocks. Some tape formats support multiple partitions too.
However depending on the tape format things might be easier. For example, the QIC-150 format uses fixed-size 512-byte blocks. And it's common for all blocks on the tape to be the same size, with a single filemark between runs. Do you remember which backup program was used to write the tapes?
A few years ago I dumped my original AMIX tape in AmigaOS, I used one of the tape handler packages from Aminet. Maybe BTNTape? Can't remember for sure. There isn't a standardised way to represent the structure of a tape in a single file, so I just dumped each file, naming them 00, 01, 02 etc. on disk.