I am in the most fortunate position of owning four quality headphone amps , my HE500 runs very well out of two , ok out of one and plain bad out of one .
Reading back in this thread I would think the bad one would be the least powerful and the best the most powerful but that's not the case .
So many options exist in todays market with different configurations and capabilities
All we can do is recommend from experience , ruling out amps or a manufacturers entire range based on specs or rumor is counter productive .
My favorite amp for the HE500 is the WA7+Tp , in terms of specs it is third in line amongst the amps I own for the HE500 with marginal power , high impedance and distortion .
What I am trying to say is best isn't always best and there are many factors that go into amp choice