More specifically and in simple form, if you unhook the speaker in the Line 6 amp and install a 1/4" jack on the wires coming out of the amp (Be sure to mount it securely somewhere in the chassis so that it can not ground it self out by falling against the metal chassis or internal speaker.) , you could then run a standard 1/4"-1/4" speaker cable to a Marshall cabinet (providing the cabinet is wired for 8 ohms or greater). You could also then put a 1/4" plug and wires on the internal speaker for use when the Marshall cabinet is not desired. Do not attempt to run both at the same time. Lowered impedance may damage the amp, and it will not sound good anyway.