I think I can explain why this happens. Do you remember the cellphones that appeared 3 years ago? Me neither... Technology moves too fast, so... those Sharc chips that the HD500 had, are not in production any more (most likely cos there are more powerful chips out there now.
So, the X, is not really a new model, but an upgrade. It's pretty much like replacing a Tegra 2 processor for a Tegra 3. Why would you try and get a Tegra 2 nowadays?
20% might not be a lot, but if you can live without reverbs (or use an external one) then you must be able to use Dual Tones more often, and that it's important for some.
This is not a real new model, this is not the HD1000, this is just the replacement of the HD500 which components might not exist anymore.