You probably know there were some hum issues with the Tascam DR60D, particularly whne using it with Canon cameras.
It happened in two situations: when screwing the redorder to the camera and when using the same battery to power camera and recorder.
Are there similar issues with DR70D?
There are lots of "issues" with the DR-70D but I have not heard that hum is a problem.
You'll probably get more responses in the main DR-70D discussion thread.