Uhhhh.... I think people are getting mixed up. You can't plug a preamp straight into a speaker cabinet. It just doesnt have the output to drive a speaker. Full stop.
Now, if the said cabinet is Powered... then yes. But as there are hardly any companies out there making powered guitar cabs (tech21 is the only one that springs to mind, only cous it was reviewed in this months guitarist), i dont suppose you have one of those.
So in short. yes, you need a poweramp before the cab.
S