What voltage and gain should I get to pair with the 250ohm Beyerdynamic 990?

It's really a nice little system. I don't need anything more on the Solid State side of my system. I have a couple of Schiit pieces. Currently working on old school side panels made of beautiful...

Still going strong after 4 years + stepping on it, well minus the cover :P

Drop everything you're doing and go to your streaming account (I prefer Spotify and Tidal) and pull up (search for) Ulrich Schnauss & Jonas Munk then find track 7 (Chasing Rainbows). Strap in and plug in your amp, and hit play. You can thank me later! No streaming account? Try this YouTube link: https://www.youtube.com/watch?v=x_mE0TE9a-Y&t=1715s

Great pair of desktop speakers! Pairing with a DAC will really bring out the best in them (not required but if you have one, try it out!). Great value deal here from Drop.

The wires on the left channel plug, just before the socket are acting up and I need to replace the cable for my Sennheiser 6XX headphones. Looking at Amazon, I see actual Sennheiser cables but one of them does not have the removable 1/4 plug (I use a DAC with 1/8" jack), and I would rather not add even more connection unknowns by using an adapter down to 1/4", and the other Sennheiser cable is far too long for my usage (looking for around 2 meters or just a touch longer. The 6XX have been around for a while and I'm sure I'm not the first to need a replacement cable. I'm actually surprised that Drop doesn't offer them perpetually like they do with the ear pads, which ironically my stock pads are holding up just fine. I'm not looking for $100+ cables here, just a solid replacement for what the 6XX came with. Thanks!