Consider this a preview, since I've only had one evening to listen with the new modifications so far. Also, I don't post many impressions, so I apologize in advance if none of it makes sense... lol.
First off, when looking at the modded GH50 Mk2, you'll know that the mod has been performed because the fabric covering in the earpads is gone and the white mesh material that John places over the driver is exposed. One of the bass ports is now uncovered as well. John also offered to replace the pads themselves, at cost, with an upgraded version from Kennerton that has a slightly denser foam inside and what appears to be a softer leather on the outside. I recommend taking him up on this offer, should he present it to you, as they conformed to my head a bit better and feel a bit less bulky than the stock pads. I haven't tried swapping them back to the standard pads, so I don't have any comments on sound differences yet.
For reference, my listening was done on the following setup: Intel NUC PC connected via USB to a Schiit Yggdrasil (Analog 2, Unison USB) which feeds a Woo Audio WA5-LE. The tubes in the WA5-LE are Emission Labs 274B mesh plate rectifiers, NOS 1952 Sylvania 6SN7 "Bad Boy" preamps, and Emission Labs 300B-XLS for power. All my audio files are in FLAC format and ripped from my CD collection. I use JRiver Media Center with no mods for playback. WA5-LE set to low impedance and low power.
Aside: One of the first things I usually listen to when trying out new gear is The Cure's "The Head on the Door", an album I'm intimately familiar with. In particular, the third song, The Blood, has a Spanish Guitar that can point out whether a particular setup has enough energy to properly convey the fast strumming and enough detail retrieval to pick up the brushing of the individual strings. I listen for instrument separation of the castanets, bass guitar, keyboard, and drums. A lot of times, the bass guitar will get drowned out by the guitar and kick drum or sound muffled on headphones with too much mid-bass, echo/reverb, distortion, or simply lacking energy and/or separation. I want to hear it clearly when it starts up and I don't want it to sound like a single tone. The keyboard should come through clearly as well (harpsicord I think it is) without piecing my eardrums or sounding shrill. Vocals should be up-front and center on this song, which isn't as easy as it sounds, since Smith tends to mumble some of the lines.
The first thing I noticed when listening to the freshly modded GH50 Mk2 is that it's easy to pinpoint where sounds are coming from, and the soundstage is wide. And it's not just wide for a closed back... It's wide for anything out there. My comment about feeling like I was listening to a pair of speakers vs headphones was what I was getting at with that statement. I think John is on to something big with his materials that direct the sound and control reflections, all without losing any frequencies in the process. It's actually pretty wild for me to think about it. With the new pads and having John adjust the headband to increase the clamping pressure slightly (it was a bit too loose stock), the drivers are actually closer to my ear, yet sound more open than they did previously.
Instrument separation is spot on. It was already good. Now it's great. Everything is present and has its own space. This is a quality that I look for in headphones more and more. If something is missing or getting pushed to the background, I really notice and just want to take the headphones off and switch to another pair where I'm not missing anything. Going back to the bass guitar on The Blood, not only was it clear and present, but actually had a bit of power and bloom. You know, what a bass guitar should have and most headphones suck at reproducing. That's probably thanks to the "big" bass on the GH50. I'm sure anyone that's heard them knows they're pretty bass heavy. Before the mod, it was sometimes a bit overpowering in my prog and core metal music, and not always accurate, like maybe one of the band members had swiped an amp from Spinal Tap and cranked it to 11. Loud, but not really controlled. Well, at first, when I put on the Mk2, I heard a lot of bass and I was a bit nervous, thinking I was going to have to plug that bass port right away, but as I adjusted to it, I noticed that it wasn't booming like before. I think John has got it tuned to where it really elevates the bass presence without getting in the way and overpowering the rest of the frequencies. That was a concern I had voiced to John before doing the Mk2 and was something that I felt needed addressing. It's subtle, but the more I listened, the more I noticed it was consistently cleaner and less in my face. I haven't done any deathcore listening yet, so the verdict is still out on that...
I want to say that I know what the metal circle behind the driver is doing, or if it's adding some sparkle to the highs and adding staging, but I honestly have no idea. Without a way to do before/after testing, it's just another bit of magic that John added to the mix. I'll have to let him address that in more detail. In my listening yesterday evening, I felt that all frequencies were present and accounted for. I didn't notice anything off or out of place, and do keep in mind that I'm sensitive to treble (I have mild tinnitus) so I focus on it immediately if it's bothering me. Part of my choice of 300B tube amp and mesh rectifiers is to keep the treble under control.
So, what does all this mean? Well, my initial impressions are extremely positive. I think this mod is essential for anyone that really likes the GH50 JM Edition, but wants a bigger soundstage, and not just bigger, but an open feeling that gives each instrument better presence and separation. Also, if you like the bass of the JM Edition you're going to love this mod. It's all still there, but more controlled. Proper power, fullness, and quantity, when called for. These two upgrades combined really gives the Mk2 a speaker-like quality that I'm not getting in any of my other headphones. It's pretty darn unique.
If you're on the fence or not sure if you should do it, I'd recommend just going for it and getting John to do this Mk2 mod now, before he comes to his senses and starts charging more, haha. He did say it's all reversible, just in case you don't like it. Also, his turnaround time was super fast, so you won't be without for long.
I'll try to offer more/better impressions as I get more listening time. So far, I just wrote about the things I immediately noticed and felt excited about, upon hearing them.
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