Buy Wavelab 9 Pro and the license should support Wavelab 6. Then you can download Wavelab 6 from Steinberg downloads Downloads | Steinberg (under Unsupported Products). Then you’ll be able to use Wavelab 6 or Wavelab 9.
At least that’s the way it worked with the Wavelab 8 Pro license. I would assume that the Wavelab 9 Pro license supports Wavelab 6 too.