http://taperssection.com/index.php?topic=170743.msg2142975#msg2142975--Will said he set his 60d output around 5/6 and his video camera around 35. I think I had my 60d output around 8. You'll want to experiment a bit to see how much output from the 60d versus how much gain on your camera. That should help.
The neat thing about the 60d is there's a separate gain control for the output to a camera and another gain control for the headphone output. I really think it is underrated for how useful it is with these physical controls compared to some of the other recorders. I think there's a way to use the slate tone to set the levels, but I suggest looking at the manual on that.
I'm forgetting what he said, but one of the guys who reviewed the A6400 said there was something different about it to deal with the overheating issue.
edit: In this video, the reviewer thinks the newer processor in the A6400 will solve the overheating issue of prior models. Incidentally, he got 1:38 hour of video recording time before the battery died, and didn't experience any overheating. It's roughly 14-15 minutes into his video when the battery finally dies.
https://www.youtube.com/watch?v=oBK1jHwdfRgm/43 caught on because the smaller m4/3 sensor didn't heat up was much as an aps-c size sensor, didn't have the 29 minute recording limit, and was affordable back in the day. For example, the g6 didn't have the 29 minute limit. Then they started putting the limit on the lower priced cameras, and kept raising the prices on the ones that didn't have the limit, or at least that's the impression I have.
If the A6400 doesn't overheat with its aps-c size sensor and if it can record for a long time using external power, then that would be a bit of a technological breakthrough and put pressure on the sales of m4/3 sensor cameras.