Right. I'm still a bit concerned that my inability to cross-calibrate between the VU meter and the deck may have left it in limbo-land, but the recorder does now seem to be internally consistent and is roughly aligned to the standard I was using on the Revox.
So, after lining it up with sinewaves, I threw a .WAV file at it, to see what it did. This is the result:
A807 recording
Original WAV file
I remembered afterwards that the A807 version has also been put through a stereo expander (on its way into the recorder), so disregard that.
One thing that does concern me is that I had to increase the repro gain on one of the channels quite a bit more than the other to get them to match... This leads me to wonder if the repro head is out of alignment in terms of height (they are pretty much phase-aligned). Visually it looks like the tape is dead-centre over the butterfly, so I'm not honestly sure.
I have not touched the bias or treble settings yet, which probably accounts for the unusual bass response in the tape version. Traditionally I've always been afraid of screwing up the bias, though with the A807 I have the advantage of being able to copy the original settings and re-enter them if it goes pear-shaped.
Any comments, suggestions etc would be helpful. I'll also try to post some video later. And the packing...