I was just wondering if you could take a small amp (for example my Line 6 Lowdown 110) and plug it into the cab of a stack without using a head.
No, because it doesn't have a speaker out jack. You CAN plug it into another amp, which would then power the stack. Any amp head or standalone power amp would work.
Hi Squierbassboy. The nature of your question suggests that you may not have a copy of this: LowDown® Studio 110 User Manual Please read it carefully before you experiment with your amp. (IIRC one unfortunate posted here last week about how he tried to power an external speaker cabinet with his combo's headphone out. He fried his amp. :scowl Reading owners manuals, and asking questions BEFORE one tries to be 'creative' with their gear (as you've done here this morning ) saves time and $.