Mikey, as you've gathered from the posts above there are a number of possibilities, and the internal design of the specific components can be an important and unpredictable variable.
My instinct, though, would be to simply use an rca cable between DAC and preamp, and to use a Jensen transformer between preamp and power amp. I'm sure that a call to Jensen will get you some good advice as to selection of a particular model.
I would not recommend using an adapter between DAC and preamp, in part because as I mentioned above most adapters short the signal on xlr pin 3 to ground (most equipment can tolerate that on its outputs, but some cannot), and in part because I doubt that it would provide any benefit compared to an rca-to-rca connection (aside perhaps from saving you the cost of a new cable).
If you want to try an adapter at the preamp output instead of using a transformer, using the adapter in conjunction with your present xlr cables,
here are two low priced examples of the kind of adapters that would be suitable. Cardas and others probably offer higher end counterparts, although keep in mind Ralph's comment above that the Cardas adapters he has seen do not ground xlr pin 3. When adapting an rca output to an xlr input, you DO want pin 3 to be grounded.
Regards,
-- Al
