Coming back to this... I installed another XM portable hooked up to the empeg's AUX input. Just like before, I had to jack-up the AUX volume in Hijack.

Is this because the empeg expects an amplified source (headphone jack)? If so, is there anything I can buy that would make the line-out match the level expected by the empeg? The Hijack trick doesn't produce the cleanest audio (but XM's stream isn't sounding too great these days either).