No, the jack on the USB speaker amp is for headphones. You can plug headphones into that jack if you have the Cube USB speakers.
The Cube does not have an audio-in jack.
The Cube amp/speakers uses a lot of power from the USB jack and must be plugged into the Cube itself. Using an iMic, or something like it, with amplified speakers, will reduce the load on your DC/DC converter and make your Cube last longer.