I disagree with the comment you would likely bypass the Stax for something higher-end after hearing the Koss. While the Koss cans and amplifier are very solid and deliver sound at a great price per dollar ratio, the Koss amplifier is not in the same league as the Stax, in my opinion. That said, the Koss is one of the most underrated headphones, period. I prefer the sound of planer and stats to dynamic headphones in nearly all incarnations. I think the HD700 and 800 are overrated for their price and require substantial care in choosing an amplifier. When a suitable amplifier is found it is most often expensive, discrete or tube based. I would take, in nearly all situations the Koss stats over my Sennheisers paired with my previous high-end tube based headphone preamp and separate amplifier. When the Koss goes on-sale around Christmas or on Massdrop they can be had with included amplifier for less than the HD700's and not much more than a pair of HiFi Man planer cans. I have seen the Koss sell for South of $650.
I have several pairs of cans for different types of music and for different moods. I found that a pair of AKG Q701's, an iFi DAC and iFi headphone amplifier (total all-in cost of about $500) delivers great sound that rivals many other more expensive equipment I own. Stats are not for everyone, though. They have a unique and accurate (generally) sound that may not work with most streamed music or low-resolution content whereas dynamic cans almost always work much better for that content, though that is a generalization and not always accurate.
The stats paired with a $500 turntable and $150 phono preamp will eat the lunch of many other systems and gear for pure musical enjoyment, at least for me. Either way you look at it, the Koss is a superb bargain at the usual $999 price and a legitimate steal when found on sale or through Massdrop. The truth is simple, if you find enjoyment in music with a $2 pair of buds and your iPhone, great. That is the point. Enjoyment. If you want what many consider to be world class sound and moving from or to a reference grade kit then the Stax are high on the list.