Nightbird95, it is the springs in those cases that causing the high resistance, AFAIK. Hi resistance shows up when you try to use high current. With X6 you use about 5A on high while with SingFire SF-90 somewhere around 1A (guessing). If you would measure voltage at battery and driver terminals at the same time you would notice big voltage drop with higher cuurents (ie. higher modes at X6). At X6 voltage drop is so high the driver thinks battery is exhousted.
I've learnt something lately about those problems. You can also see the thread where Garry tries to improve current over his driver or latest posts about DX driver you are using in X6.
Edit: In addition to that there is problem with connector resistance. It is problem with all connectors of that type. No big deal while you run at low currents, but with higher ones over 2A resistance become very limiting factor. See thread at DX forum.