I have the original "Auri" wireless surround sound headphone system. This is the system that is currently being sold under the Sennheiser name as the DSP Pro.(at Audio Advisor, you can get the DSP Pro and a pair of HD 580s for $200)
I prefer this system over the Sony or the new Sennheiser RS-60, because you can use your own headphones. Plus I disagree with the use of open-air design headphones with these systems. You can hear too much ambient noise with them( anybody out there with noisy New York radiators), I much prefer the closed air design. I use either Sony V-900s or MDR-CD 1000s.
As for the sound quality of the processor. I think the Toltec processing used by the Auri/DSP Pro is just outstanding.
And we must remember, we're only talking about a, at most, 2" driver in headphones. So while claimed frequency response may reach down to 5 Hz, it ain't the same as having a 12 incher pumping away with abandon.

So, if you're interested in this sort of thing, I suggest getting the separate processor and buying some good headphones.

Paul