The v1.5 uses carriers. I guess they can adapt the end to thread on to the regular Q8 head. If they use the same emitters, 3v xpl, then the carriers have to be in parallel. So you have the same voltage, but more run time.
Of course carriers can be modded to give different voltages. If Sofirn made a head that used 4 x 6v xhp70.2 then they would probably make the carriers 6v and they would still be in parallel.
This is probably a topic better discussed on the v1.5 thread.
Maybe Sofirn can give more info on the extended tubes.