Looking at pictures of Pioneer's XM equipment makes me think this is not a "stereo headphone jack." It seems there is nothing screened on the modulator to indicate what this port is used for. If it were a line-out I would expect it to say it on the equipment or in the specs because this would be a valuable feature. The only XM tuner I know of that has RCA outs is Sony's.

If you want to check, I would plug some cheap headphones in there and see what happens. It's better to harm a $2 pair of earbuds than your empeg or your car's speakers. I would expect this port to be some sort of data port (probably serial) for future controllers or remotes.