How does the Koss ESP950 compare to the entry-level STAX SRS-2170, which can be found for a whopping $400 USD cheaper, new?
They each have their strengths and weaknesses...
The ESP950 have better bass impact and more upfront midrange, but I feel that the 2170 has more detail, better highs and larger soundstage. Feel that 2170 has more of a flat even response but 950's are more fun.
Comfort wise personally I prefer the 207 as they allow for more movement and have better pads (950's can't swivel at all and has cheap vinyl pads) but 950's are lighter and is slightly more roomier inside the pads.
Build quality wise, no question, 2170 is miles better than the 950 system. But then again, do consider that the 950 system has lifetime no question asked warranty while 2170 only has a year, and is tricky to claim here in N. America unless you buy it here through Yamas in which case it becomes a lot more expensive.
The 950's also have a unique advantage in that it's design allows it to be easily transportable... It comes with a good carrying bag, headphones are modular in design so it can be separated into three pieces, and E90 amp can even be powered by a battery pack that it comes with.
So it really depends on a lot of factors on what your needs are. If I had to choose one right now I'd prefer the 2170 system as they're comfier (to me anyway, may differ for others), has flatter response and is cheaper to get new, but others may value the 950's more for their more involving sound and excellent warranty policy (plus 950 system can often be had for much cheaper than MSRP if you look around).