Getting More Heat To Basement?

I have a finished basement, but it gets cold! We have the thermostat upstairs set to 68 and it can easily be 55 - 60 in the basement during the winter. It's fine in the summer, though. There's only one register on the supply side and one on the return, servicing about a 700 sq. foot area, both are in the ceiling. My duct work was designed for a 2 ton unit which is located in the basement that I'm trying to heat. The unit itself can be switched to 3 tons, but it's currently being ran on the 2 ton setting.

Is there anything relatively easy that I could do to get more heat down here? I can figure out the duct calculations, but my gut tells me that putting in more registers and switching it to 3 tons isn't feasible due to the fact that the air handler is directly into a 16x8 duct. I'm currently running a small space heater that works well, but it's costing me a fortune.

The insulation in pretty poor. The walls have standard 2x4 insulation, probably around r-13, but the rim joists have almost none to speak of. Maybe I would get some value out of redoing those and making sure there's a thermal barrier up to the interior walls, which there currently is not.