You're English is great - no trouble understanding .... but some difficulty giving an honest definite answer because I'm relying on my memory rather than doing an AB test.
For your scale, I'd say 78. I'd say the difference between usb power from my netbook and a 9V battery was 75 to 76.
I haven't tried any other xo's yet. I've ordered one I know has less than 1ps jitter. It is about US$5 and it is 3.3V so I'll hack in 3.3v from elsewhere to make it work. The idea is that this will receive better power than the 5V xo when the device is running from usb.
It'll also mean that only the line driver (for the I2S) is running from 5V, and this chip can handle lower voltages, so the whole device can then be powered from 4 x 1.2V AA nimh batteries. Then I'll be able to switch between usb and battery power, and if the new reg is good, then I shouldn't be able to hear any difference.XO91
AFAIK, I have to use 12Mhz because this is the clock frequency for usb signals. If I popped in a 22Mhz or something it either wouldn't work or would play at the wrong speed (?). USB is 12Mbps so a 12Mhz xo makes sense