Using both the DAC10/ST10 and DAC9/2x STA-9 combos in the same system there are differences between the two but its really a matter of degrees imo rather than magnitudes.
Speaking purely for sound reproduction if someone were to ask me which ones they should get, my response would be based on how well recorded the music they typically listen to is. Classical and other genres where there is great production value to me the DAC-10 is the hands down winner. The quote from the review that the DAC-10 is more detailed in the treble area but without any harshness is pretty much the best description ive seen for the DAC. Most of the music i listen to is mid-80s to early 90s alternative, which certainly doesn't meet the threshold of well recorded and can easily come off as harsh. i never found that with the DAC-10 which is impressive where there is such detail, even if you don't necessarily want to hear all of the detail. As you said the musicality of the DAC-9/STA-9 it great for that type of music where hi-fidelity was never the aim by the artist/producer/engineer. Again not huge but noticeable differences. For the albums I have that are well produced the DAC-10 really shines. When it comes to sound reproduction i don't see either as the clear winner in all circumstances. The DAC-10 is better all around but the DAC-9 certainly can shine in some cases.
I will say when it comes to other quality such as looks, build and the like I do think the DAC-10 is a step above. I personally love the design and the thing just feels so solid (along with a remote that is probably considered a lethal weapon). That isn't to knock the DAC-9 as at half the price does comes some compromises. For their respective prices, and honestly compared to much higher price points, both are great in that regard, the DAC-10 ST-10 just offering a bit more in that area.