I can't stop loving this hp, but it is still not popular (yet). So I just want to give it a fair justice, and will come back and revise later.

Since it is a quick review, and many readers just want comparison, allow me to compare to HE6SE and Ether Flow since they share a few things in common.

Let me present my setup:

Gustard X16 -> Adcom 545 -> Adcom speaker switch -> GL 1200 or Hifiman HE6.

TL;DR: It blows HE6SE away.

There are only few discussions about GL 1200, but in this forum (as well as many others) there are enough discussions about the HE6se and praise how great the sound it is (arguable one of the best in sub $2k range). I know, HE6se is sold at $650, but this is not a reason why it is bad. It is the reason you should grab one!

Getting back to GL 1200. Since I already have the setup, the only thing I want to mention is the volume difference: I used -30db for HE6se, and -20 for GL1200 to get the similar sound level. Note that HE6se is super inefficient, What does it tell you? Yes it needs power to drive GL 1200.

HE6se has one of the widest sound stage in its class. Yet compared to GL1200, it is still in the 10th row in the concert hall. I mainly listen to classics, and I can tell the difference between

1) Paganini's 24 capriccios by Perlman. I was literally in the 3rd row of Perlman's live concert a few years ago and this is the first time I feel I am back to the seat. HE6se is more like the 10th row, but more importantly narrower soundstage.
2) Strauss's Zarathustra by Reiner / Chicago. Sunrise! Smooth and grand!
3) Random Access Memories by Daft Punk.
4) Mozart's most famous Serenade 13 by Camerata Salzburg / Vegh. Vivid and live. HE6Se sounds like a narrow studio compared to GL 1200.

Basically you feel you are around the musical instruments yet still enjoys the whole scene. As a a comparison, DCA's Ether Flow Open can pickup details, but over time you feel they are disconnected and bored.