I am very familiar with the Druids, having listened to them at a friends house. He bought them new, and they took many hundreds of hours to fully break in, with bass being the most noticeable during the process. His bass is so good, some have even asked, "Where's the subwoofer?"
Have you experimented with the gap between the bottom of the speaker and the floor. This definitely affects the bass response.
Also, the impedance curves of the Druid and Vandersteen are quite different. A speaker cable can work great with one speaker and not so well with another. Myself and a few other friends have experienced this with various speakers and cables.
Have you done anything with your power, dedicated lines, upgraded outlet, power conditioner?
One thing to keep in mind is that the sound of Zu speakers is extremely coherent, and that will be compromised when you add a subwoofer.
BTW, the MF A308, great amp, I have one as well.