The normal version is working in ACSS current mode and is tuned for a neutral sound signature, SA version works in diamond differential and does not use ACSS nor ACSS cables, instead has only RCA and XLR input/output and is tuned for a "musical" sound signature. I don't have any experience with their diamond differential musical dac's and amp's, but from what I read, it is possibly something like "neutral, but slightly sweet" (google translation of audio-gd's chinese sound flavor page), the mild degree of coloration mainly noticeable on piano and violin but makes vocals more enjoyable. Speculation on my part.
Some people may strive for the most honest reproduction and any coloration is undesirable, but there's some people who would not mind a little coloration if it can make the music more enjoyable.
I wouldn't worry about the volume. You can ask Kingwa to lower the gain for you, so that shouldn't factor into your decision at all.
My personal experience with Audio GD stuff has been their DAC19 DSP1, one that they consider neutral. IMO, it's fantastic with instruments, but vocals don't stand out at all with it (not that it does vocals badly, it's just not what you'd buy it for). Stuff I've read says that their musical stuff is generally better with vocals. So, if you listen to mainly vocal music I'd get the musical stuff; if its instrumental stuff I'd go for the neutral. Keep in mind I wouldn't go for the neutral stuff just because it's neutral; accuracy doesn't mean **** when your systems so boring to listen to it's essentially a series of paperweights worth over $1000.