Whole lot of non sense, your (or Mine) HD 650 do not have a "ultra smart AI and magic circuit" that will know which amp it's plugged into, when it pluged into my prehead it's not going go "I am in prehead now, I will draw 500mw, but instead of making it really loud and blow the driver I will use the extra power to make it sound better!!" Or "I am in STX now, I will draw 30mW to get the same Vol but it just won't sound as good."
At same wattage, the same headphone it will have the same vol, 500mw likely will kill it or make you deaf. Headphone amp is not about pump out more wattage, it's about how well it outputs at certain wattage.
If you want to run a flyweight amp with a modest (at best) switching power supply out of your computer, fine. Just stop pretending that it is something it isn't.
If you want to go into the timeworn automotive analogies, all this modding, etc. is just putting a bunch of performance tweaks on an economy car. If you go to the trouble of building a linear power supply and installing proper caps and resistors, it will no longer fit in a computer case and will cost substantially more than buying a desktop amp.
Me? I wanted a sports car. I didn't buy an econobox and put a ton of crap on it. I went and bought a RX-7 and leave it stock. The RX-7 cost less than an econobox with a bunch of crap and it performs just fine. That's the same reason I don't use tweaky lightweight amps - I start with a proper one. You don't have to do the same, but give some serious thought to starting out with what you actually want instead of pimping out some consumer grade card that's engineered for a 12-18 month lifespan.