So, a friend of mine is selling me a Marshall AVT50 (50 watts) Head for $250(which is a pretty decent price), but I don't have a cab, so I was wondering if I could my 150 watt Line 6 Spider III combo as a cab without hurting it. A friend of mine told me I could, but I've got a bad feeling. So what's the deal?
i have the same amp (line 6). it wouldnt work unless you rewire it, because there is no power amp input, only 2 75w outputs.
you know, ive owned 8 different guitars over my lifetime, and right now i have 3. i wanna know what the hell happened...i mean the picks and cables are understandable, they disappear into thin air all the time, but guitars?

You cannot plug the head into the input of the Line6. (which seems to be what Rhoadsfan92 is talking about.)

What you can do (if you are willing to drill two holes, solder 4 wires, and spend $8 on parts) is install a line out on the amp portion of your Line6 and a speaker input jack for its speaker. If you want to do this, I can provide you with instructions.