There is a problem with the 8611A.
If you load it to the maximum torque it will burn out the motor very quickly. Fortunately its very hard to get it to the maximum torque. About the only way is to get it stalled in some way.
The 5955 uses the same motor but the amplifier limits the current to a safe value. On the other hand the 5955 will not produce the torque they claim for it.