Haven't posted on Head-Fi for quite a while (not that I ever posted much...), but I did feel like I should offer my brief impressions on the revised (now MKII) GH50 JM Edition I received back from John Massaria a week ago. I'm not good with any of the technical terms I'm afraid, so this will be a very "layman's terms" impression.
The first and most noticeable difference between the original JM edition sound and the mkII mod, IMO, is the depth of the soundstage. To be honest, my immediate reaction was disappointment, as it seemed to have an echo like effect. Allowing my ears to acclimate to it over the past week, the "echo" is still noticeable but not as bothersome as before and the added depth is a very welcome and impressive addition. Width-wise, there is an increase as well, but to a lesser degree. As a whole, the increased soundstage gives a greater sense of space while still retaining the bulk of the forwardness and agressiveness of the GM50's sound.
Bass quantity seems to have increased a notch but is more defined. There's more texture to the bass than prior. As much as I liked the GH50 JM's original sound signature, I did feel the bass could at times be boomy and not defined. Even with the added quantity to the MKII version, the bass doesn't give me that impression at all. IMO, this is due to the increased depth of the soundstage, as the bass is not as forward as before. The one critique I have here is the bass still lacks punch.
I'm not really detecting much change to the midrange. It's every bit as meaty and forward as before. This is the area I was most concerned with, as it was the midrange I liked the most of its original sound. I am very glad that if there is much difference, I certainly am not hearing it (or recalling it).
Treble has a definition bump. It is neither extended nor rolled off from what I can gather. It's easier to pick up the details of the treble than it was before. Again, I believe the majority of the change is due to the increase of width and even more so increase in depth of the soundstage.
Final verdict - I'm very happy to have sent my GH50 to John for his mkii revisions. To me, it's an upgrade in the areas the GM50 may have been lacking without taking anything away from the best elements. Many thanks to John for his first GH50 revision (the JM edition) and his continued pursuit of getting the GM50 to reach it's full potential (mkII). Kudos, John!