Thinking about getting stax headphones.

You'll definitely need an energizer or an amp to run stax headphones. You'll need a "pro-biased" output to run the one in your picture. However, there are some self-biasing transformer boxes spritzer made, which are very small and portable. I'd say it's close enough to a converter-cord. PM duggeh, he might still have it. 

If you really don't want to deal with all the troubles of finding an amp, I'd say get a pair of used HD600 or HD650 here on the forums. They offer very close performance to the stax headphones I've heard. Sounds great straight out of a laptop too.

Because of how differently they operate, electrostatics need specialized driver units. They can either be cheap transformer boxes that need speaker power amps to feed them, or direct-drive amplifiers that tend to cost just as much, if not more, than the headphones themselves. I think that's the main barrier to entry for electrostatics; at least with dynamics and orthos, you can just plug them into anything and at least get some sound, even if underdriven.

However, it's not impossible to get Stax setups at a relatively low price, otherwise I wouldn't have two Lambda setups by now. You just have to keep your eyes peeled for the right deals.

The issue with Stax headphones is that they're electrostats.  Electrostats have two stators with a thin diaphragm stretched between them.

You have to put a good amount of power on the stators for them to work.  A few hundred volts for headphones and the electrostat speakers will put a few thousand on them.  

Soyou have to have an amp that can charge the stators.  A computer soundcard is not designed to do that.  Speakers have the stator supply built into them, which is why they work with regular speaker amps.

I'd recommend a pair of good dynamic headphones that will work out of your soundcard.

The closest you can get to electrostats without requiring a Stax amp or a transformer box in my mind are orthos. That would be a Hifiman HE-4, HE-5LE, or HE-500 depending on your budget and preferences. Alternatively, you could buy the $75 Fostex T50RP, and have it converted into a Thunderpants. None of these will work that well straight out of a soundcard, but a $100 amp should drive them perfectly well, and you really can't get away with spending that little on the electrostatic side, not unless you already own a speaker amp to drive an old Stax transformer box.