If the sub provides speaker-level inputs (some do, some don’t) you would connect it to the speaker terminals of the 219ia, together with the connections to the main speakers.
If the sub provides only line-level inputs, since the 219ia doesn’t provide pre-outs (line-level outputs of its preamplifier section) the likeliest alternative would be to connect the sub to the Oppo. Either RCA outputs of the Oppo, using a splitter if necessary (so that those outputs could be connected to both the sub and the 219ia), or XLR outputs if they are provided.
You would then have to adjust the sub’s level control each time you change the 219ia’s volume control, to keep them in sync. Or, alternatively, if the Oppo provides a volume control you could use that control to change volume, and perform a one-time determination of a specific volume control setting on the 219ia and the sub that would always be used with that approach.
This assumes that the sub provides inputs for both left and right channels (if, as it appears, you are using just one sub rather than two). If you are using one sub and it only provides an input for one channel you would have to use a mixer of some sort to sum the left and right outputs of the Oppo together, with the output of the mixer being provided to the sub.
Another possibility is that you might be able to find a suitable speaker-level to line-level converter. I’ve seen such things offered over the years, at low cost, although I have no knowledge of their technical parameters or their sonic quality.
Regards,
-- Al