The tubers do not have any growth points on them so you do need to take a cutting from the stem. They do not need tubers in that case.

Thanks I was misguided at least 40 years ago

Monty Don did this on Gardeners World and i think it was the the last one. He took a cutting from close to the tuber but only used the stem. As Owdboggy says.

Thanks everyone

I used to work for a professional Dahlia grower and around now he used to take hundreds of cuttings from the new growth on the stems. When they were rooted he used to throw away the old tubers altogether and start with the fresh plants.

An interesting thought

Thanks to everyone