You know, as you mentioned, I would not be surprised if Magneto is for PPC, not for x86. It probably isn't a Universal Binary like the new stuff.
Hmm... yeah for free... maybe try Nebula Free? They have a mac version I think. And it is cheap for the Commercial one. Lots of tape emulation in there and according to many, among the best.
Personally I use Magneto, Ferox, Ferric TDS, and URS Saturation. All are different and good for different things. If I had to pick one, it would probably be URS Saturation. Many options, all sound good, and pretty low on CPU.