In general, connecting the Sonos Port to a free tape loop would be advisable. I am not familiar with your specific device, however.

Could you use the Zone 2/Zone B Line Out port on your Onkyo?

Thanks both- I tried Zone 2 line out, but it doesn’t seem to work.

I’ll try the free tape loop- need to get the right analog adaptor

thanks again!

There doesn’t appear to be tape in/out on that receiver. Such concepts must be too ancient for it. 

You should use the Line Out (Zone 2). There may be configuration required to enable those jacks.

Edit: See p45 et seq of the manual.