XStreams must be "slam opened."
You can't slowly open the tank valve the way you were taught for EVERY OTHER REG.
The reason is that the servo chamber must get pressurized to close the valve. If you open slowly, the XStream (which delivers such huge flow) keeps the dynamic IP from rising enough to pressurize the servo, to inflate the valve insert, to close the valve. The reason is that all that gas is going out the mouthpiece faster than the first stage can keep up, with a partially opened tank valve.
XStreams all "PSHHHT!" loudly for 1 second after opening the tank. If that continues longer than 1sec, see @iointerrupt 's post #3 above.
They are great regs, and certified to 600 feet! There's a reason they are used by several nations' military divers.
But they are...different.
