I was about to pull the gun on this Sennheiser cable: https://en-us.sennheiser.com/accessories-connection-cable-3m-jack-35mm-hzl-26-withhout-adapterThe only way you will find out is from the experience of doing those things That steel cable is .. well .. steely in it's sound, especially in the upper mids, a touch edgy with some music, though I've heard worse cables over the years. I've certainly had many years of great joy even with the steel cable and original pads, and I still have that original steel cable with it's DIN plug/adapter to 6.5mm plug on the end (I gather for balanced operation at the time), every now and then to give myself some perspective I take off the Mogami and plug in the original cable ... it doesn't stay long. That upper mid grainy edge appears immediately and lacks some body to images. In saying that, when I bought the cheap Sennheiser replacement cable, made with it's thin copper that just felt 'lacking' in quality, the sound smoothed out a bit alright, but so did some finer detail and openness, while sounding a little lifeless and boring. I went back to the steel cable at the time very quickly and stayed with that tattered item (the channel wires had separated some years back from constantly dancing around in sheer pleasure and occasionally pulling the cables from the cans ... none of which has damaged the headphones themselves - this happened many times over the years!). It was only when I used the Mogami cable on my Denon AH-D1000 cans (the famous Merkel mods elsewhere in Head-Fi) that I thought to make some cables for the Senn HD540's. I've never looked back (other than the brief re-establishment of my decision to check that I had actually upgraded by moving to the Mogami from the Steel cable by going back to the latter from time to time).
The stock HD650 cable is better than both the steel and el cheapo Senn replacements, while the Mogami is better for any Senn I've had in house, including the HD6xx I had for some months a few years ago. They just didn't have the sheer openness and organic fluidity and timbre of the HD540 Ref 1's, despite marginally finer midrange detail.
This is of course my own experience and may not be that of others.
So now I'm guessing I'm better off sourcing the parts (the Mogami wire and the connectors) and a capable iron to put my own together. I have never used a soldering iron before, but hopefully this sort of procedure (DIY headphone cables) doesn't require too much skill.
I can't believe you mentioned the grainy upper mids, because that was exactly my impression of the HD520. I think the upper mids are a little strident at times (strangely enough, more so than the treble), and I figured it might be just the character of the cans, or maybe a combination of that and the pads with fosilized foam. But perhaps the steel cable is playing its part too.
I'll post some more information about HD540 Jr. (haha) some other time, but one thing I notice is that the bass is more solid in the HD520, plus it is very well controlled (not boomy at all). Not that I find the HD540 bass lacking myself, but the 520 has more of it. The mid and treble is not as good as the HD540, but it is very good still. And the soundstage on the HD520 is huge also, though I am not sure if it is very accurate, because sometimes I hear sound from behind my head, which is a little puzzling until I let go and enjoy.
I just played that Nicola Conte track that @T1000 mentioned earlier, and wow! Having headphones that are neutral and have a great soundstage is such a treat, specially for acoustic stuff.
Another track I listened to earlier which is amazing for the soundstage and instrument positioning and dynamics is the Basket Game cue from the Indiana Jones soundtrack:
Not surprised that you pulled the cables in sheer ecstasy a fewtimes over the years. It's good to know these headphones are tough on top of all!