The K712 has a pretty good size cup, I can measure it for you later today if you'd like.
I don't have the magni but others that do have said it is adequate for the k712's.
Unamped the k712 doesn't sound very impressive. It's not horrible but it benefits greatly from an amp.
Yeah - that'd be great. I found some screenshots of the PC360 Ear Pads (70mmx43mm) : http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/rG4AAOxy9eVRMGiM/$(KGrHqNHJE!FElO+iUdKBRMGiLjLE!~~60_57.JPG
The DT-990 is 53mmx53mm.
Have you tried the HE-400? I would be interested in seeing if there is more ear pad room in the HE-400 or the K712s.