The 1216 is a guitar driver. That's what the G at the end stands for. That doesn't mean that you can't use it for bass, though. However, the 1216G doesn't have a lot of throw (Xmax), so it can't move a lot of air. You won't get a lot of low bass out of it, and, if you were to try, you'd likely damage the drivers. They'd make some ugly sounds first, though.
So, if you like the sound you're getting from them, go ahead and use them. If they start to sound stressed, back off on the bass or volume, or maybe both.