6v when you're not even cranking?
A low voltage reading at the starter, with 12v+ at the terminals of the battery, would generally indicate a bad cable. The cable is becoming a load in the circuit and 'using' voltage. But it shouldn't happen when the starter is not being triggered...your B+ cable should have no path to ground, and thusly there is no current flowing. And you can't have voltage drop with no current flow...you will always read battery voltage to the point of an open circuit.
When you turn the key, the 12v wire to the solenoid gets switched on (should have no voltage with the key off), and that completes the connection between the big B+ terminal and the guts of the starter. Current then flows through them and ground is found through the starter housing and the engine block.
Are you sure you have the starter connected right? You didn't accidentally connect the solenoid wire to ground, did you? I suspect not, as you would have either immediately blown a fuse when you turned the key...or set the wire on fire (if the circuit is not properly fused).