[DAZZER]

Long time ago I saw on the Head-Fi forum probably the best version of the famous HD25 for sale, a unique custom Sennheiser HD25-13 II Aluminum (600 Ohm). The 600-ohm drivers have more details and less distortion than other HD25 series. Unlike plastic, aluminum cans reliable, and not boomy. I really wanted to hear how they sound. So I decided to assemble the same ones but with some changes. I made an Uber mod (mass load) and removed the foam filter. The weak point of all Sennheiser HD series headphones has been removed (loss of contact at the cans). Therefore, the cable now is not changeable - this is Klotz TP414 (made in Germany) with a gold-plated Amphenol jack (it's easy to resolder to a balanced connector). Finally I upgraded noname memory foam pads to Yaxi Pads.
Here they are:

 

[MakeshiftApe]

So how do they sound compared to the 70 ohm w/ plastic cups?

I actually heard that with the HD25 the 600 ohm models wasn't really much different sounding than the 70 ohm models (although I could be remembering wrong), but I have heard good things about the aluminium cups being better than the plastic ones. Big fan of my HD25 - built like a tank, passive isolation that has me still not having bought a single ANC headphone since why bother when I can passively block out even the noise from a train with those, and a fun sound signature that has enough of everything to be good for just about any music without ever feeling insufficient, and no bothersome frequencies (to my ears) to make it fatiguing.

I actually got mine by mistake. Many years earlier I heard a friend's HD280 Pro, and wanted to get a pair myself since I enjoyed the sound - but I mistakenly remembered the wrong name and ordered a HD25 - glad I did!

 

[DAZZER]

Nice story!
I guess it has more flat response with more detailed and analytical sound. If you're in Warsaw, I can take them with me sometime if I'm there. Then you can compare it personally. But need to find an adequate amplifier.