I got a 1/4" TRS female to 1/8" TRS male adapter with HD 650 from Sennheiser, it was included in the case. May have changed since then, but it was a standard accessory with HD 650 earlier at least. It's pretty nice too, it has a pigtail so you don't put a lot of strain on the small female jack on your amp/pc.
Thanks for they reply, Norway. I decided that if I don't want to get the big amp then I should back off from the big headphones. Looks like I'll be giving the 598s a try.