The ZYX output figures are based on a different velocity (I think that's the parameter). Not sure about the technical details, but it results in a real world output that is quite a bit higher than the spec would suggest. You may want to talk to Mehran @ Sorasound who will give you the "skinny" on that. In the $2K range, you might be surprised at how far up the ZYX line he can get you. Disclaimer- Never heard a VDH cart.
