TheScorpioI have had the one linear($150 which is much more reasonable price to performance) and currently been running the 789. Both amps sound very similar and I have not had a Magni 3 but the reviews state it sounds very similar to both drop thx amps. The one linear is twice the cost and has less power than the Magni 3 Hersey. The Jds atom doesn't offer much in difference in sound and has less power as well. I can't think of any amps that will give you the performance to dollar and sound the magni 3 offers. If you don't need the power th Magni 3 offers the Jds atom is good choice if don't stay with schiit products.
TheScorpioI've had the Atom and now have one of these. The big difference is desk space and build quality, the case is really sturdy and heavy compared to the plastic Atom amp. They both sound crystal clear and anyone would be hard pressed to tell the difference. Depends on if you need the additional inputs for the Atom, the power and voltage at 600ohm is virtually the same though, and quite a bit less than the Heresy.
I've had the Beyerdynamic DT990 600ohm and they will power them and planars too. Unless you have headphones that need the additional voltage of a Heresy then you can't go wrong with either. I have the One amp with a JDS Atom DAC (the AK4490 version) and they look great in a stack.