I used the new firmware with the "Advanced" tab to adjust the clock frequency down to 150mhz, with that the consumption goes down to 240-250w on the wall ( at 310 Gh/s ).
The resistors are located on the back, or the inside where the big sink is attached. You will need to completely remove the board to get proper access to the resistors.
The default value is 2.083 Kohm. And I use the pencil mod to bring the value down to 0.900 to 1.050 Kohm. I have most of mine set close to 0.950 Kohm.
After that the consumption will be down to 180w on the wall ( still at 310 Gh/s ). Noise from the fans and temp will be way down after this mod. And you can use a single PCIE cable to power one S3 ( two hashing boards )