The only way to make a basement comfortable is to take control of the Return air of the HVAC system. When I design systems for houses I like to take at least 40% of the system air flow capacity from the basement with a register as close to the floor as possible. This will help with lowering the humidity in the basement Help prevent mold) and taking that free cooling from the basement floor and cool your house without turning an the AC, and warming the basement in the winter. Dont get sucked into thinking that Warm air registers are what make you comfortable. Properly sized and positioned Return or cold air returns will make you more comfortable then trying to regulate the supply air registers.