The pre-outs on a receiver are analog, and you would feed that analog signal into the analog RCA inputs of the 08 Pro.
That's exactly what I am doing -- and it doesn't work. There isn't a twitch on the meter and nothing is audible. It isn't a cable problem, I tried several. (Edit to add: It isn't Bluetooth being active either, I tried all this before I tried Bluetooth at all.)
I know my A08 Pro's RCA input is working because if I hook up another analog signal, like the headphone output from another gadget, there's no problem.
I have scoured my receiver manual and as far as I can tell, the preamp outputs should always be on. There are no settings at all for their operation. That is why I was wondering if the preamp outputs are intended to be lower than line level.
The other zones are a good idea, but sadly my receiver is so old that zone 2/3 are nearly useless. They can only provide a signal from an analog source!
I just found a forum thread discussing the same problem in another Sony of similar vintage and the poster did discover that the preamp outputs were not as hot as line level. So, I am optimistic that the preamp I ordered will fix this.
Thanks for your patience everyone, I did not intend to execute a lengthy threadjack.