I realize this is an old post but thought this may be helpful to others trying to use URC Dimmers (600M/6MLV) for LED Lighting.
The minimum load requirement for these dimmers are 60 watts for the 600M and 40 watts for the 6MLV. The 6MLV may dim LED's better because of the lower minimum load requirement. Theses dimmers do not need a neutral wire.
Just an FYI, the URC switch S6AM has a minimum load requirement of 10 Watts and a neutral wire is needed.
The higher the load (watts / number of bulbs) of the LED lighting you are trying to control, the better your dimming results will be.
Using a Lutron LUT-MLC (resistor) will let the led bulbs turn completely off and help with dimming a little (got me to about 50% before flickering the bulbs). If your only problem is the bulbs not turning completely off, a LUT-MLC will fix that. Others have also suggested using a 1.5 kOhms ±1% 25W Wirewound Chassis Mount Resistor (RH0251K500FC02 / KAL25FB1K50 )
Using a Lutron LUT-LBX Synthetic Minimum Load For use with Reverse and Forward Phase Dimmers will let the led bulbs turn completely off and help with dimming better than the LUT-MLC. The LUT-LBX got my LED bulbs dimming down to about 15% with no flickering. EBay is your cheapest bet for the LUT-LBX. The LUT-LBX requires a double gang box for in-wall or surface mounting, but can be added anywhere after the dimmer switch and the first lamp.
I did some bench testing with a 600M dimmer (I did not have a 6MLV to test) and a LUT-LBX with about 5 different dimmable LED bulbs from different manufactures and different styles to test the minimum load needed for the bulb to dim without flickering and turn completely off. All bulbs dimed down to about 15% with no flickering and turned completely off with 16 watts of load. Some of the bulbs went as low as 6 watts of load before a slight flickering.
So if you want to use LED lighting with Complete Control (CCP) and a 600M or 6MLV dimmer with good results, you will need a Lutron LUT-LBX.