Most regulated devices have what's called "boost and buck" circuitry. That means it will
boost the battery limit higher or
buck it lower. It allows you to put in a freshly charged 4.2v battery/cell in your device and vape at 6.4v or 3.7v. (or whatever volts are required to produce the watts you set it for in VW mode).
The IPV D2 can only boost, it cannot buck. So with a fully charged battery/cell it cannot give you less than 4.2 volts. You can get less voltage as the battery drains.
This doesn't apply to the equipment you are using. Buck regulators are absolutely necessary for batteries wired in series, which adds the voltage of each cell together. (2 batteries/cells in series = 8.4v) A buck circuit does not require having to use any of the batteries store power to meet the set voltage, it just restricts it's output (for lack of a better term). So if your in series batteries/cells are 8.4v and you wanna vape at 4.0v, a buck circuit is required.
Buck-Boost Converter
You're good. You can use the Joyetech Delta 2 RBA base. You don't need a different mod.
Regarding online purchasing, I often use
vapeCrawler to see who has the best pricing. If I only want to see suppliers from the USA I make sure USA is "green" and all the others are "gray". If you go down to the very bottom of the page you can see all the
vaping websites vapeCrawler searches.
I keep this thread bookmarked:
Deals and Steals | E-Cigarette Forum.