I hooked up the high level connection to the A2080 in the lounge earlier and ran some REW tests. It seems that the receiver isn't doing what I expected.
I'd assumed that when in Pure Direct mode that ALL processing would be bypassed. In that it would send the full signal through to the mains despite them being set to small in the YPAO settings. But it doesn't, so the mains have nothing much to send via high level - thus scuttling my cunning plan.
I was comparing various bits of music on the stereo, straight and pure direct settings the other evening and it seemed that in some areas (e.g. string section on classics) that pure direct sounded much cleaner. Though trying it again today and not much difference really. Might have been confirmation bias.