Why do you want to keep the 1070 in there if you get another AVR? You also have the 1080 power amp, right?
It makes no sense to have two receivers together. But if that's what you want to do, then you'll need to set, and not touch the volume on the first receiver with the digital input. Where you set the volume is another thing. You're looking for the unity gain point (which doesn't technically apply to digital inputs). That's were the line-outs are delivering at max, the max input voltage the 1070 expects to see for max output.
It's a messy hack. Not many receivers have line-outs, only the higher end ones. Those will more than likely meet or exceed the performance of the 1070 itself. Or you could go with a straight pre-pro instead of a receiver. All of those will also be of the level of performance you're getting from the 1070.
