If you have 6 wire motors, the motors are probably wired internally for unipolar drivers.
Each set of 3 wires that connect to each phase has a center tap that attaches to the High voltage.
The other two wires drain two oppositely wound coils that drive one phase of the stepper motor.
If you want to drive it with a bipolar driver like Picstep, you must only drive one of the coils on each phase. The second oppositely wound coil cannot be connected as it will buck the force of the first coil.
You can find the center tap by measuring the resistance of the coils.
The center tap will have half the resistance as the measurement across both coils.
Connect the Picstep driver across only one coil for each phase.
You will find it runs smoother and has much more power.
Otherwise, you need a Unipolar driver such as the SLA7062m chips.
You cannot drive a 6 wire stepper motor "serial bipolar"
If you want to drive "serial bipolar", you must open the motor and separate the center tap feeds into two lines and bring them out of the motor.
This will give you an 8 wire motor which you can drive serial bipolar.
That will give you the maximum power out of the motor.
A discussion of this is at the following thread
http://www.cnczone.com/forums/showthread.php?t=14287