I'm also a DC-3 user, and I have tried a variety of configurations. Here are my observations:
4x12: I have run the amp through a Marshall 4x12, as well as an old Boogie 4x12. I do not own either of these cabs, but have used them at venues where backlines are provided (Both closed back). I felt the Marshall 4x12 overall sounded bigger and better. Hooking up a DC-3 to a 4x12 completely changes the amp's sound. The bottom end is HUGE, it bites nicely in clean settings, and the overdrive gets incredibly creamy. If space and weight are not an issue for you, I would HIGHLY recommend you try this.
2x12: I use an older Fender open back 2x12 cab when we play in larger rooms, and disable the internal speaker. I run out of the 4ohm speaker jack. This also makes the amp sound very big. Present bottom end (not huge sounding) is very cool, mids seem to get more scooped sounding, and the highs are sweet. Because of the size and weight issue, this is my preferred approach.
Internal Spkr + 1x12 open back: Doing this gives you a cool option - split the speaker cabs up so that have one speaker stage right and one stage left. It doesn't sound the same as using the 2x12 cab, but is definitely a good sound. It's basically the sound of your Boogie doubled. You probably won't notice a drastic increase in the bottom end of the sound, but the mids do seem to pick up a little doing this. The top end sounds a little bright sometimes.
Here's a link: this is a live recording with the DC-3 running into the 2x12 I talked about above. http://www.jam-bone.org/sebastianos/sebastianos.htm
