Apparently, USB high-speed mode can handle 1,5MB/s which is enough for 6 channels of 24/44.1 (which is almost 800kB/s). However, 6 channels of 24/96 is over 1,7MB/s and thus too much for USB.
Ego Systems have a 2I/O USB audio interface which has 24bit converters but only supports 16bit due to the low transfer rates of USB. What gives? Maybe Ego Systems don't know what they're doing or the faster USB standard wasn't released when they made their box. This seems strange as well though as the lower speed standard handles 187kB/s, which is enough for 2 channels of 16/44,1, while Ego System claims 2I/O with their box.
Confused yet? Yeah, me too.
/Ola