how does rogue zeus compare cat or atma-sphere?

If I recall correctly your speakers although sensitive dips below 4 ohms.This would create a potential problem with all but the very best high powered,high current tubed amps.
Be sure to listen to all candidates in the comfort of your listening room.Best of luck

they actually remain flat at 8 ohms

If they remain flat at 8 ohms I would consider the atmasphere ma1 mk2.3 or the ma2's (more power) very seriously; it may be worth a call to talk with Ralph from atmasphere; he is a great source of info and won't jam his gear down your throat; as this has been my direct experience.