Thanks guys.
The speaker does cut out when something is plugged into the "pwr in", so I assume I can just pop in a 1/4" plug without it being connected to anything and safely cancel out the speaker sound?
I would be using my aviation headset, Telex 150 ANR, I don't know if having something like that would make a difference over regular headphones. I haven't had a chance to try it because they are in the plane and I haven't gone to get them yet. The impedance is 150 ohms mono, 300 stereo.
From what Curtisa has said, I gather that it would be best if I did 1/4 to 1/8" from the pre-out and then into a stereo system and then use the headphone jack on the stereo system? That shouldn't be a problem to do.