Rear ported, yes, although the rear port is actually the end of a 1/4 wave transmission line. Whether or not a passive radiator is a better solution is debatable, in the end it comes down to execution. They do work reasonably close to a rear wall, about 10" minimum distance, I'd say. They do not have minor bass issues, they just need some care taken over set-up, the same as the Shahinians.

I did a lengthy thread on the Heeds over on pfm. Heed say that they had the Arc to hand as a yardstick during the development of the Envoys and wouldn't have released their speaker if they didn't think it bettered the Arc in every way. As Alpar is a very open and honest man, I'm inclined to believe him.

Jason Hector reviewed the Envoys in Hifi+; he preferred the Obs, but those are quite a bit more expensive than both Arcs and Envoys. Having heard his system, I think I know why he prefers the Obs, and in his circumstances I might well come to the same preference. But your room is a good bit larger than the one Jason is using, and I'm not at all sure I'd prefer the Obs in a larger room.

Still, it's your ears that would have to be the arbiter.