I assume by Lusty, you mean Mad Lust Envy?
The Astro Mixamp is frankly a little overpriced, especially if you're using it with a PC. The Mixamp uses Dolby Headphone for surround processing, so does the Xonar U3. Neither of those two have really great amps (so for challenging headphones, you would be best served with an additional discrete headphone amp anyway), though both should be sufficient with the CAL!s. With the price difference between Creative's X-Fi Go! or Asus' Xonar U3 versus the Astro Mixamp, you could probably put the savings towards a nice amp and be better-off overall. Lastly, the Mixamp requires Dolby Digital Live from an optical output, so a DTS Connect signal wouldn't work. Computers can actually detect the Mixamp as a USB sound device, but from what I read the USB isn't well implemented.
As far as "Lusty" saying the CAL!s work best with Dolby Headphone, a more accurate way to look at it is the CAL!s are better-suited for surround processing than most closed-back headphones because the CAL!s have good soundstage for closed cans, and the best processing method (Dolby Headphone, THX, SBX, CMSS-3D, etc) is the one that works best with your ears. Your ears and your brain process sound "directionality" differently, use the videos in the guide in my signature to make up your own mind about which works best – I don't (yet) have a SBX or THX TruStudio Pro sample, but you can hear Dolby Headphone and CMSS-3D.
Right now, I'm happy with my Recon3D USB processor, I like THX better than DH or CMSS-3D personally (though I recently heard a CMSS-3D sample that impressed me, and logically I think it should be better and more refined than DH), and I like that I can use the USB with my Mac/PC and the optical connection with my Xbox, though the built-in amp alone isn't sufficient for my Q701s and the DAC isn't as good as a dedicated PC Card or a high-end USB stereo DAC. Nameless has a Recon3D USB too (unless he sold it already?), which he didn't like as much as his top-end Titanium HD soundcard and Stax headphone setup (understandable) and found that the Recon3D USB doesn't natively support surround with some retro gaming titles. The Recon3D is designed closer to the market segment of the X-Fi Go! And the Xonar U3 — I wonder how it compares to those, though the Recon3D obviously adds external control buttons, Optical surround input, and part of a wireless adapter for one of Creative's other headphones besides the CAL! (Creative Aurvana Live!, the same company as makes the X-Fi and Recon3D products).