The Dolby Headphone chip that the OP describes is an extremely old version that Freescale (who manufactures the silicon for Dolby's chips) phased out about 3-4 years ago. It's at least two revisions behind the current one. This older chipset had three different surround "Modes" : DH1, DH2, and DH3. Nobody really used any of the modes other than DH2, so Dolby simplified the chip and began shipping a product that approximated DH2 (and just called it 'Dolby Headphone'). DH1 didn't contain noticeably enough surround information (echo and reverb) to matter, and DH3 was just obnoxious. DH2 was what everyone used 99.9% of the time.
The other big benefit of using this new chipset is that the MIPS (instructions per second) are lower, and it uses about 35% less power (if I remember correctly). When you are running on batteries (or on the outer edge of the USB power envelope), this is a huge benefit. (affects the MixAmp in both cases).
The current chipset that we use today (two newer than the old revision that the OP referred to) supports Dolby 7.1 input (matrixed rear side and rear back channels). it's an extremely subtle effect and we don't jump up and down about it here at ASTRO, but the technology is there. It uses even more efficient architecture, so we can give the amp even more juice.
If you are in Japan, you can get a better version of the JVC SU-DH1 -- it had abysmal sales according to Dolby due to it's poor amplifier/power output, no provision for a DC/wall power input, and general lack of quality.
The Japanese version is the Victor SU-DH1 -- which has DC-in (yay!). This product also has DTS decoding, which is a benefit to people that are really in to Home Theater. For gaming though, you are going to love the MixAmp. Gaming is where the ASTRO stuff is really going to shine.
As for the A40 Headset, you've already gotten some really good advice. The only thing that I would add is that in my opinion, two full-sized drivers along with the top-grade Dolby Headphone technology in the MixAmp give you the best of both worlds: 5.1 when you are plugged into a full sized console, and full-range stereo on the go. It's really nice to have a full-range headset when you want to use it for anything other than gaming on your console--say on an iPhone, PSP, or DSi. Music and games through your portable system come to life because of the full-range driver arrangement. You simply can't get the same performance with only 1 out of 3 tiny drivers working in a multidriver headset. Plus: the A40 makes bass that you *hear* -- it doesn't simply vibrate your ear with a transducer. Being able to use a headset with only a single device limits its value to a large degree -- even though it is "cheaper".
BTW, don't forget to plug in the A40's mic when you are using it with your iPhone and PSP (PSP-3000 and newer). These types of devices have 4-pole inputs that are fully compatible with the A40's standard cable!
--Stormy out.
