This is only my five cents of guessing, so take it for that.
The limit to 96kHz on USB is most probably because the box uses a slightly older USB 1.0 chip on the connection to the USB bus. The limit on USB 1.0 was 96kHz, while the newer USB 2.0 goes higher. This makes the box compatible with many computers (mostly Windows) that would need installed drivers to use USB 2.0. Long story though, and I do not have all of it.
As for 96kHz or 192kHz there has been a lot of discussions on the benefits of each. The main line on the areas where I tend to hang around, recording engineers, seems to be that on playback it mainly depends on the actual hardware used. Some boxes sound subjectively better on 48kHz, some on 44.1, some on 96 and some on 192. You simply have to try them. The difference tends to be very subtle and 2 persons seldom agree which is better. A small side note is that all modern DAC chips upsample digitally to higher frequencys inside the chip in order to be able to simplify the analog circuits (becoming both less costly and better).
Modern DAC-s generally sound good on 44.1kHz 16 bits and there seldom is any reason going above. The limiting factors generally are the combination of speakers + room. (From the examples I have seen, most audiophiles tend to be severely limited by the lack of treatment of room acoustics).
The difference in playback on 16 vis 24 bit is basically that 16 bit has a little bit more background noise. There is seldom any reason to go for 24 bits in playback. When recording we use the extra headroom (difference between noise floor and signal) of 24 bits to be able to more lazy when setting levels.