I wouldn't say that I am an expert in this field (or even in these forums, seeing as this is my first post), but to my knowledge and experience, it is not wise to turn on your source after your amp is on (with headphones connected).
The correct procedure would be to turn on your source, then amplifier then plug in headphones.
Powering of the audio card (and power loss) in any device will create a popping pretty much every time. If I were you, I would only plug in my headphones after I knew that everything else was on. I have experienced this second hand and I know that to much popping at excessive volume can cause your drivers to blow.
The same happens to me when I reboot my source and forget to turn off my Bryston amp, even when it's not loud, hearing PMC IB1s pop is a very scary experience.