For some reason, I never have the patience for manuals, but thanks to Gonk, once again I had a look and to my surprise, found what I was looking for on the first opened page:

"The Model 990's multi-zone capability works only with analogue audio sources,"

which pretty much answers the question, as you all have.

That means I need either to supply an analogue connection between the 990 and 981 (as explained here) or find another method of attaching my external headphone amp.

In looking at the manual, it displays the rear of the 990, showing at # 19 Preamp audio outputs. These carry all 7.1 signals, but would it be alright to attach just a pair of stereo leads for my external headphone amp?

thanks,

John