My Astro A40 mixamp arrived yesterday. First, it comes with every type of cable you could possibly need, except the optical TOSlink (cost?). I plugged the (included) USB to USB-mini cable into the back of the 360 for DC power.
Plugged my ATH-A900 into the headphone jack and fired up Mass Effect. The first thing I noticed was how darn quiet the background noise was compared to the JVC adaptor. There was none.
Next, I jumped into my saved game. Then, I almost jumped out of my chair and turned around because I thought someone closed a door behind me. It was Mass Effect. Unbelievable. Again.
There is definitely some room for improvement. But, compared to the JVC / receiver combo I was using, this is yet again a big jump. There's still a gap right behind me, but it's good enough. The mixamp has plenty of headroom to drive the headphones directly, so there's no need for an additional amp. Good volume level, almost too much at max setting. It's set about half-way.
formulanerd - get a good set of diapers