About your choices... I don't think the U3 has full functionality with OS X, in fact I'm pretty sure it just would show up as a generic stereo device. So probably no DH from that. Second, I've been very specific to NOT ABBREVIATE Recon3D USB, because there are Recon3D internal soundcards (no native Mac drivers for these by Creative) and then there's the USB unit that does have Mac drivers and CAN address DDL input from consoles S/PDIF optical so that consoles can supply surround sound. Third, the Recon3D USB does not have a digital output to connect to another DAC... the Recon3D USB >is< the DAC, and you would want to double amp using the analogue headphone jack. Don't worry, this works pretty well because the noise floor of the Recon3D USB is pretty low, I've had good/great results with all my amps.
Creative makes an X-Fi Go! Pro (or something similarly named) that is the equivalent USB stick as the Asus Xonar U3, but it says on the product page that it doesn't support Mac either. OS X may still mount it as a generic stereo device, but that doesn't give you a benefit over the pretty good internal sound on Apple computers (most of their headphone jacks also double as optical outputs, too).
I've mentioned before, Creative is rumored to be making another Mac-compatible external sound device, but I don't know what that'll support and I bet it's at least a few months away.
Yeah, here's where I think you're saying that you think the Recon has a digital out... It doesn't. The E17 is a DAC/amp, and the Recon3D USB is a DAC/amp with only a headphone output, the E17's amp section is stronger than the Recon3D's but neither is really good amping for the K701 by themselves. A receiver with headphone surround (like the old Denon/Marantz receivers with DH, old and current Yamaha receivers with Silent Cinema, and other manufacturer's implimentations) would have a decent amp section to be an all-in-one solution for the K701, you can get one like mine for less than $200 US dollar. If you use the Recon3D USB + an amp, that amp could be a FiiO E09k or E12, Little Dot i+, or one of the amps Head-Fi user Chicolom has enjoyed with his Q701 (Matrix M-Stage, iFi iCAN). Some people like the Objective2, but I think it's the person child of a desktop and portable amp that borders on analytical sound and was designed to prove a point and make NuForce look bad; it's a good value SQ-wise but I probably won't buy one.
Does that help any? I'm falling as.eep, and typing/concentrating is getting hard when some of my blinks starts some REM activity. Goodnight my friends on Head-Fi!