So I got my ST and H6 today and got them wired up. Installation took a few tries (the power connectors on the Asus cards sometimes need reseating before they'll properly detect as being plugged in), but all in all it took less than an hour from shut down to newly operational. The difference in sound quality from the DX is subtle but noticeable.
As observed by Inflatable Mouse, the headphone output will not function while the 8 channel output is also functioning. I also can't find a way to flip the switch between the two in JRiver, I only seem to be able to change it over in the Asus Control Panel. Which is really odd, because JRiver seems to have no problem overriding other ACP settings in ASIO mode, but it may just be something that only the driver can do.
I've only had a few hours to listen to it, but I have three primary technical observations:
1) The output is slightly hotter than the DX, and I had to reset my JRiver reference levels. The difference is two or three dB.
2) The ST is (as I've connected it anyway) very slightly higher noise than my DX. I say this because while the DX produced no audible noise of any kind in my HF drivers, the ST produces soft audible noise in one channel only, but no noise in any of the other channels. The noise isn't audible (over room noise) from more than a foot away. One thing I observed when originally connecting the DX was that, depending on what else was connected to the same molex as it was, I got a different noise floor, and I experimented until I found the "zero noise" molex connector. Because the ST and the DX take a different pin connector, I couldn't just use the same "no noise" connector, but I tried two or three different connectors and picked the lowest noise one I found so far. I'm going to try and troubleshoot this and will edit the post if I find a way to eliminate the noise floor on the ST. This is one of the unusual features of the Asus cards, i.e. that they have a separate non-PCI (or PCIe) power supply and are very sensitive to noise in that supply, unlike the M-audio and creative internal cards I've used in the past (which had far different problems, don't get me wrong).
3) The sound quality is definitely better, and I'd have to say that my sub-bass and mid-bass both sound significantly "tighter" and punchier. I'd say the effect on the total package is that the sound is "airier" and less muddy overall. My DX (and now the ST) was connected directly to my power amps (i.e. I'm using the sound card as my pre-amp), and I almost wonder (for this and other reasons) whether the DX had a high enough line voltage/output impedance to drive my power amp.
Regardless, the ST sounds clearly and noticeably better, but probably not enough better that I would recommend the ST/H6 combo over the DX to a budget-conscious stranger who needed an 8-channel dac given the price tag is more than triple. That said, incremental improvement is still improvement, and every audiophile tweaking endeavor reaches a point where improvement becomes increasingly marginal. I'll be hanging onto the ST :-)
I'll edit this post if I find a molex that cures the noise floor or if I encounter stability/driver issues at some point (I don't expect to, but you never know with PC audio). If anyone needs a DX for testing or use, I've got a spare that needs a home ;-)