Hmmm...the switch my Prodigy came with has a resistor in it...unless I'm totally confused.
My switch has a little blue ceramic tube looking thing. It turned dark when the switch quit working so I sent it back and PS sent me a new identical switch. I didn't know there were "unresistored" switches. I thought the new mechanical switches they will be offering soon were the ones without resistors.
Anyway, whatever switch it originally came with (with the little blue ceramic thing in it) works just fine with either two RCR123A 3.0 V batteries (for 6V? or 5V? whatever it is) OR with one 18650 3.7V. You shouldn't have to buy anything extra -- just a different battery.
It works great for me, anyway.
Sounds like you have the resistored switch, so two 3.0v batts give you around 5v, but a single 3.7v only about 3 or 3.2v. The non-resistored switch would vape much better with the 18650 3.7v battery.