I just finished my first ever Magnum build just over a week ago, and I'm really happy with the results! Ordered the "Turbulent X" drivers from Turbulent Labs, but I thought their cups were too expensive and I ended up finding a better deal on eBay. I was really happy with the cups, so I thanked the seller and asked him whether he makes them. Turns out, they were made by a head-fier named 7keys, and he sells the cups for him. Whoever 7keys is, thank you!
I also modded my old SR225 set - gave it a single detachable chord, and threaded a fairly thick Canare L-4E6S cable through the headband and into the other cup. Needed to create extra holes in the cups for the wire, but it worked out beautifully and I really love the set. Most single chord mods I have seen for Grados create new holes in the top of the cup, which prevents it from swiveling properly inside the gimbals. I created a hole in the side, so that the wire leaves the cup right next to the spot where the gimbals hold the cup, but on the inside - between the gimbal and the pad. Since the hole I created is right next to the swivel point, the wire is not in the way of the gimbal and the cup is able to swivel completely normally - the wire just comes out of the back of the cup and goes up into the headband.
This is my first ever post so it's not letting me put up pictures. As soon as the mods approve, I will upload some pics for you guys!