I have a partially finished basement that is connected into my house's 2-ton A/C system. The humidity in the space is consistently high during the summer (60%rh and up), and I believe that this cool, moist air being drawn into my HVAC system is making it work harder and inhibiting it's ability to cool/dehumidify the living areas (which are uncomfortable on hot days). I don't really care if the basement is warm, though I do wish to control the humidity so nothing gets ruined and to keep the cave cricket population down. 

What I've tried:

1. My first inclination was to cover the basement return vent, but the return duct is huge (16inch diameter) and this made me worry that I would drastically change the pressure dynamics of the system and possibly damage the blower motor if I disconnected it. Of the 3 return ducts in the system, it is by far the largest - the others are each only 6 inches (this weirdness confuses me).
2. I have sealed around the foundation/rim joists with spray foam to prevent outside warm/moist air intrusion, this reduced the humidity somewhat.
3. I have added two 50-pint dehumidifiers to the basement, one running continuously and another set to go off at high RH. This seems to work to reduce the RH in the basement, but I don't know if this is helping me cool the house, as it is producing hot air, all day long, that is feeding back into the HVAC return.
4. Covered the supply vent. I'm not sure if this does anything, the return will just suck cool floor air down the stairs from the 1st floor. It makes me feel better about wasted cold air, so there's that.

I have hired 3 different HVAC professionals to help me with this, and the end result is I now know 3 different ways to rip everything out and start from scratch ;) No hard feelings, I think it's funny.

Any ideas? Could I safely disconnect the supply and return?

EDIT: A little more info about the duct layout:

* the A/C system was added on to an existing forced-air heating system where each (1st floor) room has a single supply vent at floor level, and there is a single return for the whole floor, at floor level in a central hallway. Supply ducts are 4inches in diameter. The single 1st floor return is a box with a large grill, and there are two 6 inch ducts coming from that box and entering into the air handler in the basement below.
* the basement return is also a box with a large grill, connected to a 16 inch duct that connects directly into the air handler. It is on a wall at the bottom of the basement stairs.