The SVT-4 Pro doesn't have a headphone out. You "could" plug that into the power amp speaker out, but the turn on click would probably be enough to blow the headphones and/or your eardrums!
You might try the channel A wet mono line output and have the PRE/POST switch on POST, it might be able to drive headphones. Although, line level out is not intended to drive the low impedance of headphones. The balanced out is probably intended to drive a 600 ohm load, so the unbalanced might be looking for 300 ohm. A pair of headphones plugged into a stereo-mono adapter plug could be <10 ohms.
Your best bet is to get a small headphone amp, and if you want to hear the tone of your SVT-4 run the SVT-4 channel A wet mono line output to the input of the headphone amp.