Currently you can only pair things that play “gapless”.
In monophonic mode, “poweron” has a gapless transition to the hum, as do most effects in fact. However, in polyphonic mode, the two effects overlap, so pairing them is not currently possible.
I suppose we could potentially change the way this is configured to something like: ProffieOS.SFX.hum.paired=out; That would also have the advantage that it would be possible to do things like pairing in to out, or pstoff to preon.