Level match: by ear.
I find that the closed setting between my A6A and my X3700H is, respectively, -24.0 and -21.5. It is not exactly the same, sadly, but any change introduces even bigger difference.
Switch: mostly myself.
BTW, I am sure the switch itself, although mechanical, might introduce a variable (like a resistor). But changing wires in all combinations doesn't hide the difference between the two receivers, which is pretty clear to me; at least, when listening to them side-by-side.
For giggles?? I am not here to prove anything. I am sharing experience for the benefit of OP, only because I have them side by side. You can read as much bias in it and giggle as much as you want.
It is not scientific, depends very much on the environment (like I said), and probably cannot be compared to pure measurements. There is no known to me bias, though. The whole reason I bought A6A is because I didn't really like the Denon's sound after RX-871. So, I am trying to decide which to keep for myself, with the price difference being $700.
Well, in that respect, if anything, I am biased towards Denon, actually.
BTW, this site measured A6A:
We bench tested the Yamaha RX-A6A 9.2CH 8K 150watt/ch Atmos/DTS:X Receiver to gauge power output and uncover performance issues we discovered. We discuss the proper settings for best power delivery.
www.audioholics.com