Noticed something on the back of the madvapes business card they throw into shipments that confirms your point:
Battery.............. Single Coil...........Dual Coil
519,901,808...........2.0-3.2 ohms.........not recommened
eGo...................2.0-3.2 ohms ........1.5-1.7 ohms
3.7 Volts.............1.7 to 3.2 ohms..... 1.5-1.7 ohms
5 Volts...............2.5 to 3.2 ohms......2 ohms
6 Volts...............3.2-4.5 ohms........ 2.5 ohms
7.4 Volts.............4 to 5 ohms......... 3 ohms
The SmokTech Dual Coil cartos I ordered from fasttech are 2.5 ohms. Based on the chart, I need a 6 volt battery. Are there any cheap 6 volt batteries I can pick-up? Would this one work?
$14.64 X6 1300mAh Variable Voltage Rechargeable Battery for Electronic Cigarette at FastTech - Worldwide Free Shipping
i don't know about the X6 in particular but i had a similar device 1 year ago with 3 fixed voltages. to be honest i didn't like it. i might find it and post a picture. it was a 3.2, 3.7, 4.0. dunno about X6, it might be very good but i doubt it since i don't see many people use it. if i remember correctly the problem with mine was a very big voltage drop under load.