We (in my first band) circa 1983/4 used to run a 30 W practice amp into a old tube Fender Vibrochamp 50 (no dirty channel). We just unhooked the speaker cables from the 30 W tranny (cant remember the make), fitted a jack and plugged it into the fender. From Beach boys to Motorhead in about 2 seconds !

It sounded great however Im not sure that overloading the input (which was what we were doing) was that good of an idea. Im also not sure Id do it with an amp I cared about !