I'll be honest, I've never owned a biocube. But the "stock" lighting you have is a couple power compact tubes, is it not? If that's the case, then yes it CAN grow coral...but it just doesn't deliver as much PAR as LEDs or T5s. So what you're seeing is probably PAR for the course (please forgive such a horrible pun)! I would expect slower growth under PCs than any of the other lighting options.
One other thing. You mentioned the tank is 15 months old. WELL past time to get new bulbs, if you haven't already. PC bulbs should be changed at least once a year. They tend to lose their intensity and will eventually red-shift, causing algae issues. It won't be noticable to the naked eye, but trust me...the algae will know when the spectrum is more favorable to them.
Finally, are you testing and dosing for Ca and Alk? Your corals can't grow well if they don't have the necessary ingredients needed for their skeleton! You didn't mention it, so I wasn't sure.
I watched the video. Nice tank you have there! Makes me want to re-start a nano!