You can modulate the original VDB data channels with Ramp nodes:

• Create an aiVolumeSampleFloat node
  Type the channel name you want to use as the driver in Channel (like density)
• Create a Ramp node (delete the palce2dTexture node)
• Connect the aiVolumeSampleFloat/OutValue to the Ramp node/V Coord
• You can use the Ramp/OutColor output as an RGB color input in the aiStandardVolume shader (for example Scatter Color)
• You can also use the Ramp/OutColorR output as single channel input in the aiStandardVolume shader (for example Density). So you can use the original ramp modulated density data as density input in the shader.

note:
You can also use the Remap Attribute feature in the aiVolumeSampleFloat node (the Bias is extremely useful).

Cheers, D