Note: I am a certified XTX50/100/200 technician and work/own these regs. This is the way I turn my own regulators and the way they act at the store after I adjust them to the apeks/aqualung specifications.
I am not sure how much you were shown or know about the regulator so lets start with the basics. You have two ways to tune that regulator and both are on the left side. You have the dive/pre-dive lever with a + and - setting and you have a control knob. These are designed to be used with gloves on so don't worry about adjusting them in the water.
The problem you had is what I refer to as a highly turned piece of diving equipment
. Whenever I get done servicing a unit one of the last things I test is free flow. With the dive/pre-dive lever in the + position (closet to your face) I press the purge button and make sure the regulator will free flow. Then I move the dive lever to the - position and make sure the free flow stops. You can also stop a free flow by placing two fingers over the breathing hole or turning it upside down in the water (faceplate facing up). Both of these should stop the free flow condition, if they don't take it to the shop where you had it purchased and have it function checked.
Why do I want the regulator to free flow in the + position, because that is a direct measure of how easy it is to break the seal (cracking resistance) and deliver air. I want the regulator to be open and delivering air as soon as I start to inhale. If you find that free flow is happening a little too easily just give the knob on the left hand side of the regulator a half a turn twist clockwise (toward your face if the reg is in your mouth). This puts a bit more spring pressure on the seat making it just a little bit harder to break that seal, you can turn it down further but don't twist it past the stop point or someone like me has to pull it apart.
The other control you have is the dive/pre-dive switch. This isn't so much a switch as it is a half barrel inside the regulator. In the + position this barrel is out of the way of the air stream. In the - position this barrel redirects some of the air towards the diaphragm to help stop the free flow.
A few things you can test if you are worried about it.
Without the regulator hooked up a tank press the purge button, it should pop right back out. If it sticks, hesitates, or doesn't pop back out take it in for service.
Hook up the regulator to full tank, turn the tank on and test the following.
Turn the breathing adjustment knob all the way out (counter clockwise if you are looking at the knob), put the dive/pre-dive switch to + and press the purge button. The regulator should free-flow, just cover the hole and it should stop. If it does not stop even you cover the hole take it in for service.
lastly, repeat the same test but with the dive/predive switch in the - position. The regulator should not free flow and if it does it should stop almost immediately. If not, take it in for service.