HE-400 i & S versions are worth trying too as they're currently in production. The SRS-2170 is very euphonic and has a surprisingly natural sounding presentation. They're faster than any other type of headphones I've heard and are arguably in a different league. Unfortunately, I find the upper mid-range and lower treble to be aggressively forward, while the linear bass provides far less quantity than my favourite dynamics and planars. To give an indication, here's my brash EQ fix for contemporary music which some aficionados may not approve of.
It's a shame I'm not fond of the signature as the speed and realism is striking. I'd still rate my Audeze LCD-XC, LCD-2 and HE-500 as more enjoyable, despite Stax's technical superiority. I'm saving up for the Stax SR-L700 as I've read the SR-507/SR-L500 midrange can also be excessive, though I'm not expecting any more bass quantity.