Hi! The best thing to do at the moment is to use the Voxel Region Mask input to control where voxelization happens. You can paint a mask and provide it via an anchor point. This will make the voxels computed for just that area, and can stack instances of the filter for different areas to get the detail you need.
I'm trying very much to improve voxel quality and resolution, so when I am able to take some time away from my main work I will hopefully be able to finish out an update which has 4x as many voxels. This whole thing is really butting up against the limitations of what Substance can do, so development has been super challenging. But stay tuned!