I have owned multiple 650, 600, 6xx and 580s before and I finally decided to keep a 580 in my rotation since I feel like even if I sell it, I will buy another one in the future. I bought mine in a local Facebook marketplace deal and it has loose springs so I got a super good deal on it. I have kept it for 3 years ever since and I usually do not keep stuff over 1 year as I keep on buying and selling gears.
IMO the HD580 and HD600 sound really similar, so if you have either one of the models, there is no need to get the other one, they sound similar enough. HD650 is warmer and the vocal is more laid back and sounds more relaxed, and the 6xx sounds similar to the 650 to me. All these HD6 series are great value for money (especially used ones) even after being in production for so many years.
My HD580 is the late production one, and it has a bump on the HD580 precision logo on the headband, it also has the silver mesh under the pads. This is the latter production which some say sounds the closest to the current HD600 sound. There are some black paper variants with a flat logo version that were earlier iterations which some says sound different but I have never owned them before so I cannot comment on that.
But for my HD580/600, I think it is a must-have headphone in an audiophile collection. It has a great history and story behind it and I think it is affordable enough for people to keep it in the rotation. And it just sounds right to me.
The vocal on it is probably the best in class, it sounds accurate and is relatively close but not as close as ATH headphones, and honestly I don't think you can find something with such a good mid at the same price point ($500 USD MSRP headphones), and I think with the right pairing on source, the mids could be elevated to the next level. The bass is lacking but it is enough for most music and it sounds accurate. Highs are fine with them as well, I don't see anything wrong with it, it is just not going to wow you. Soundstage is very average, but again it is doing no wrong and just presenting music in a neutral manner.
With the stock cable it sounds good, and there are some decent options out there on the market for you to refine the sound. I personally am using the Cardas Clear Clone and it improved the mids slightly and made the 580 sounded slightly warmer.
The 580 also reacts well to different sources, you can play it safe and get a normal SS amp and DS chip DAC and it will sound great and accurate out of the gate. You could also get a tube amp and pair it with R2R dac and make it sound musical and warm and lush. I prefer the latter pairing, but either method is fine, it is just a matter of preference. It also scales pretty well with source, but you don't need to invest too much in them to make them sing, you just need a bit of power to push it properly (like don't just connect directly to PC or phone, get an entry-level amp so it sounds loud enough). This is why I am super impressed by them and I would recommend anyone who is starting the audio journey to get the HD580/600 or 650/6xx.
Aftermarket support is exceptional, there are many cables, pads and parts available. You can get official or aftermarket accessories. I recommend sticking with official pads but for other accessories, you can try out aftermarket stuff. I am always impressed by Sennheiser having spare parts even for older headphones like the HD580, I bought the spring directly from them.
They are super easy to disassemble and maintain, overall quality is super sturdy and mine is still going strong. My only complaint is that the driver is hard to get, if it is broken it is not easy to fix or to find a replacement. Also the wire between the spring and the driver is not easy to resolder if you broke it, it does require some specific tools to do that. They don't tend to break though so I won't be too worried about that.
The HD580/600 just sounds right, and IMO it is a great headphone as a benchmark and you should try it at least once in your audiophile journey. They sound right and is very neutral. It is not gonna give you all the micro details or provide a wide soundstage or give you a lot of bass, but it just does everything right. And the mids are super nice as a bonus.
If you have never owned or tried the HD580/600, make sure to give them a try. If you like them, then neutral headphones are for you, if you hate them then go for something that is less neutral and go in another direction instead! I find this a very good way to know what your preference is and it is also a great way to introduce others to try tube amps and different sources due to how good it reacts to sources. Overall this is a very good headphone, even for now. I would highly recommend the HD580/600 to anyone who wants to get into the audio space.
Bass: 6.5/10
Mids: 9/10
Highs: 8/10
Soundstage: 6/10
Price to performace ratio(MSRP): 8/10
Easy to push? (higher score=easier to push): 6.5/10
Overall: 8.5/10