Hum/What does X, COM, GRD mean on a Fisher SA-300

[Scottgehman]

I'm pretty sure COM means common, Grd means ground, but what does X mean on the speaker terminal strip of a Fisher SA-30. When connecting 8 ohm speakers, should I connect the speaker's negative terminal to COM, Grd or X (or am I supposed to short any of these together?)

Also, I've also got a faint hum (60 hz, not any harmonics) in channel A after the amp warms up a minute or two. I've recapped the larger caps about 8 years ago (I've left the ceramics alone.) Switching tubes doesn't matter.

Thanks for your help