HD 600

I decided to get myself an upgrade for my birthday. I was previously using a pair of HD599s hooked up to the balanced output on my KA15. This really woke them up, but didn't do anything for the clarity or the low end those were lacking in. Clarity is hard to measure for me. I have hearing that allows me to hear electricity in walls (and dog whistles, unfortunately), so I don't tend towards headsets with overly punchy highs. I tried out the HD600 and the Focal Hadenys before getting to these, and found them overly flat, and overly harsh, respectively. The sweet spot for me is in the mids, and these have such a rich and creamy mid response to them. That also aids in creating a breathtaking soundstage. There's a lot going on in a Kate Bush record and these articulate things I swear I've never heard before. Likewise, listening to Charles Mingus, every instrument has position out in front of me, like I can reach out and touch them. These also have fantastic low-end for an open-back headphone, and especially compared to the HD599s. The opening track, "Angel", off Massive Attack's Mezzanine is a great demo of this; the track comes in with an intense synth bass riff that the HD660S2 gives almost perfect weighting to without sounding boomy or weak. I think if you're looking for something with pinpoint accuracy, these aren't the cans for you, because of their extensive low and mid response, but for me, they address the issues I was having with my prior set and have really opened up my music to more exploration. The balance between the low/mids and highs on these seem tailor made for folks who don't want an abrasive high frequency experience. They're relaxed and smooth but still detailed enough to qualify as an audiophile grade experience and a huge step up from the HD5xx. I'm super happy with them.

Recently picked up some budget tube amps to test with my HD 600, the Apos Gremlin and Fosi GR70. For my DAC/solid state amp I use the Aune S9c Pro and have used the XDuoo TA-66 in the past. My main problem with the TA-66 was the shear size of it. Didn’t really fit on my desk. So, I’m testing out these two. The Gremlin is obviously a cooler looking device, with its totally exposed board, bright orange glowing tubes, and the green LEDs (that can be turned off if you don’t like it). I would say it moves the needle towards a tube sound just slightly from the S9c Pro, but sounds crystal clear, and punches really well. The totally balanced connections can kind of be a pain. I have the R70xa as well but only the stock cable currently, so I have no way to listen to them on the Gremlin currently. Also no line outputs. The GR70 is definitely more understated. It has a quarter inch and 3.5mm headphone outputs, so it’s the opposite Gremlin, as being only single ended. It does have an RCA output you can use for speakers or a different amp. There’s some oddities with the Lo Z and Hi Z for me. According to the documentation, I’m supposed to use the Hi Z mode with the HD 600, but it sounds really muddy that way. I tried all the headphones/IEMs I’m listening with at the moment, and all of it sounds better on Lo Z. Not really a huge issue, just odd IMO. The sound on the GR70 gets closer to the classic tube sound. The tone controls work pretty well, but as anyone with an HD 600 knows, there’s only so much you can do with bass bumps before things start distorting. The treble dial works great though, you can really brighten the HD 600 up if that’s your thing. If you are also considering these, I think you should think about your use case. The Gremlin is great, with some caveats: you need to have a balanced DAC, you don’t need it to be a preamp, and you don’t already have a pretty good solid state amp. I just don’t think it moves the needle far enough compared to my S9c Pro to warrant keeping both in the same stack. The GR70 does provide a different enough sound to warrant having both it and the S9c Pro. It also has the preamp options that work for me since I have active desktop speakers that benefit from a slight tube softening (Kali LP UNF). No balanced outputs is a little annoying, but typical at this price for a tube amp. The Hi Z Lo Z stuff is odd but not a dealbreaker. I’ll most likely keep the GR70 in this setup and take my Gremlin to work since it works for my setup there (Geshelli J2 DAC) and not needing the preamp options since I won’t have speakers.

IDK, but as someone who's been with HD600 for more than 20 years - it is now collecting dust and HD490 became my endgame headphone pretty much.

HD600 are not known for soundstage but are known for their realistic sound especially in the midrange. The Hifimans you mentioned are (also consider XV). Personally I think soundstage for headphones is overblown. Most of it comes from the recording itself. As far as power, you need to leave some headroom for dynamic peaks in the music which greatly exceed the average volume level you are listening to.

You're never going to be able to hear music as the artist intended it. However, neutral headphones are still really good to get. I always recommend the HD600, because they can play everything extremely well.