I just finished reading Traeger's excellent Magnus Opus on bass repair and set up... And, incidentally, I have a similar issue with my 1/2 size German laminated bass. First I should say that I am not a luthier, just a guy who really likes to know how things work and to tinker. Now, on to the bass... My bass has a slightly off-set neck, by roughly a 1/4". This causes the bridge to sit off-set in order to center the strings on the finger board. I have to compromise this in order to achieve better center footing on the top of the bass where the feet of the bridge are over the bass bar (mostly centered) and on the G side by the soundpost. What I learned from Traeger is that there are SEVERAL things that influence the sound. You and I most likely have a bass with a resonant tone around 60 hertz right where the D string resonates. To fix this, try Traeger's Mode Matching. It's all in his book "Set Up and Repair of the Double Bass for Optimum Sound". Here's a link to Amazon... http://www.amazon.com/Setup-Repair-D...d_bxgy_b_img_y
Also if you do a search, you'll find articles by Traeger on Mode Matching. But the first thing I'd try is tweaking the soundpost position. Looking at the bass with the bridge facing you (E on the left and G on the right), you can adjust the volume and tone by moving the post. Keep in mind that you always want to keep the post south of the G foot and inside the limits or ends of that foot. Moving the post North with make the D and G softer and less 'bassy'. Moving it South does the opposite. Moving it East brightens the sound, West darkens it. Traeger usually positions the post more East for pizz players and west for arco. I moved mine a little north and a little east. I then moved my bridge so that they were lined up north of the F hole notches instead of centered. It tamed the D and really brought out the dead low E. My A is still choked, but I think that is a bridge shaping issue.