I found a note in a post from last year - No 3.5mm analogue audio output option, since latest update, Pi 3B - #2 by fzinken
and have added
dtoverlay=pisound
dtparam=audio=on
to /boot/config-user.txt
This has added an analogue output device (bcm2835 Headphones) which will output audio to the amplifier. But there does not seem to be an option to output both any more.