You can adjust motor stiffness by changing the P-gain parameter. Please look at the MX motor manual LINK. It is at the address 28 (0x1C), where default value it is 32. The less P gain, the larger the back lash, and the weaker the amount of output near goal position. In EMYS head (yaw and pitch) it is good to set even 16 or 8 it depends which joint. Set 8 for discs servos than EMYS' disc will be more susceptible which cause better impression during physical contact, ex. during stroking.
Remember, this setting is stored in the RAM memory part, so you should do this operations every power reset.
You can make this configuration with Dynamixel configurator available in Download section. Unfortunately this software is not full compatible with MX series servomotors therefore use CW Compliance Slope register position and be careful during this operation. If you are using EMYS control system based on URBI, you can make this operations with these urbiscript commands:
Dyn1.SetCWCompSlope(_Head_IDs,8); Dyn1.SetCWCompSlope(_Head_IDs,16); Dyn1.SetCWCompSlope(_Head_IDs,16);
This part of code you can find in the Emys.u file. The new P-gain parameters are set every system set up.