If you connect the imac to the audio in of the FoxLO, the audio out of the FoxLO needs to be connected to your speakers.
You can keep the FoxLO anywhere. On the ground in a corner will give a deeper sound.
That device wouldn't work with the UCubes as they are USB only. They get power from the USB input and don't have any other option for analog.
The USub isn't very descriptive about how it connects. Since it comes with just a "2 meter audio cable" it probably goes in the chain between the left and right UCube speakers.