Your review falls right in line with my general headphones experience, comparing dynamic headphone with the Sundara, except I don't have the exact same amps, which I'm sure your high end amps are all superb for the Sundara. Once I heard the Sundara with my speaker power amp, I felt like this combination is the "Goldilocks" -tuned combo. There are headphones that are thick and syrupy, and there are those with very dry, clinical sound. My system sits right on the goldilocks peak of what I would call the "tuning bell curve." It is present, enveloping, and full sounding, without sounding uncontrolled, slow, and unclear. It seems based on your review, the HE-500 is also very close to that peak, perhaps erring slightly on the smooth/thick/underdamped side of tuning. What I appreciate about the HE-500's FR, compared to the Sundara, is that it is linear straight down to 20hz, whereas the Sundara does suffer a gentle rolloff, which can be easily fixed with EQ.
Comparing either of these with dynamic headphones, especially in the lowest octave, the difference is very clear. There isn't one high-end dynamic headphone on the market today that can go as low as a Sundara without distorting heavily at high outputs. With a powerful enough amplifier, you can certainly test this. I had done this for the HE-400 (original) and the HE-560 (original). They go so deep and powerful without distortion, that these planars are simply untouched by any dynamic headphone...and it's not even close. If you try to get a Sennheiser to rumble as much as a Sundara, you will damage it, at high output. Fortunately, you won't get to the level where it would damage the dynamic can...because long before you even get there, the amount of distortion is enough to tell you to stop.
