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So what you are saying, is that the amp of the xonar stx is actually pointless. Well, maybe, I've never noticed an improvement from using it (sometimes , I was even hesitating to say, if it was not worse when using high gain setting).
So what , because of the 10.7 ohm impedance, the xonar stx is coloring sound ? (well I'm not an expert regarding impedance stuff).
I don't think the sound was only brighter out of the xonar stx , but especially more refined.
In fact I would have think that the sound appear as being "brighter" because of being more refined, and not the reverse.
The amp isn't pointless. If your headphones are hard to drive, you'll need the extra volume. For example, I listen fairly quiet so even with the relatively hard-to-drive LCD-2 I'm fine at 40% volume on Normal gain. High impedance headphones require more voltage to reach higher volumes. Even at higher gain settings the distortion of the amp is lower than cheap stuff, say onboard sound or a portable player. So you are getting a cleaner, louder signal with the card's amp than something else, even many other amps. If you don't need more volume, then yes the gain options are pointless. That's what they're there for, volume.
There's also the fact that a lot of the card's processing (Dolby Headphone for example) is tied to the headphone jack and won't work with the RCA outs. If you count that, the amp is very useful.
Because of the output impedance, the amp delivers less voltage to low impedances than it would if it had low output impedance. Every headphone has a certain impedance curve, which represents the impedance at a given frequency. You can find many, like the
SRH-940, at Inner Fidelity. Notice the higher impedance at the mid-bass and high treble. Because of the amp's output impedance, it will deliver more voltage at those higher impedances than at the lower impedances. In other words, it'll be a little louder there. The rule of thumb is that anything more than 1/8th of the headphone's impedance can be audible. Not always, because it depends on the impedance curve's variance. The 940 doesn't vary too much, so it won't be affected badly, but its low average impedance (40 ohms about) is only 4 times higher than the card's output impedance. My LCD-2, for example, has a ruler flat impedance curve like all planar-magnetic headphones. So the only problem I have is reduced voltage. I don't get coloration.
It's possible the Clip+ is giving you audible distortion, or you have the volume turned down enough that the noise floor is obscuring details. I'm not sure if the limits of audible THD or IMD have been written in stone.
Output impedance is one of the reasons the Clip+ is so great. It doesn't have the best noise or distortion levels (an iPod Touch beats it out slightly in some measurements, but probably not audibly). It has a great, low output impedance though (1 ohm, inaudible with all headphones). That makes it ideal for headphones with widely varying impedances, like balanced armature IEMs.
