+1 for the Lavatube and 3.0 Boge Carto's using that @ 4.4V and loving it, another cheap option would be a BOLT + Extender + KICK set to around 8 watts.
If MONEY is no object, well you have 2 choices as far as I'm concerned, Provari or Darwin, both are EXCELLENT devices, I'd lean more to the Provari as I enjoy the tanks that I have and the darwin is kinda awkward with tanks.
But yeah, on the cheap, Ltube or KICK'd Bolt and on the expensive Provari or Darwin.
Ryan