As you can see in the picture below, I have what could be considered 5 different ways to power my small collection of headphones. I've been trying the different configurations available over the past couple of weeks and have come to the decision that I like them all but for different reasons.
The HP out on the Zamp adds a bit of bass boost to everything that's plugged into it with the exception of the Sundaras. It really helps the T90s as far as bass response but makes the D5000s too boomy for my tastes.
The Zamp, through the Can Opener (Zamp/Can), seems to have the widest separation and greatest detail which surprised me given the not-so-great reviews of the Zamp. It's driven by a Vincent SA-31 Preamp which uses a pair of mini-tubes in the circuit. A lot of reviewers consider it just to the warm side of neutral. The Sundaras really shine with this combination. Detailed and punchy - I can't help but wonder what I'd get with a better power amp.
The headphone amp in the UD-501 is adequate and capable of driving all of my headphones to levels higher than I'd ever listen. The assumed weakest in terms of power of the 5, it still sounds way better than plugging into my laptop or phone.
The WA6 is the most different - almost equal to the Zamp/Can in terms of soundstage but less analytical and warmer. I do notice a smoothing to the bass on the Sundaras when played through the WA6.
The HP out on the Rotel RC-1572 MKII is great. I don't hear any colorization, detail is there, soundstage is as good as the Zamp/Can, slightly warmer but not as warm as the WA6. The Rotel has an internal DAC while the other combinations are using the UD-501 as their audio source.
What I found most interesting was, with the exception to the Zamp HP out, how similar each system sounded with each of my headphones. While there were differences, the differences weren't that great. Everything pictured below was acquired over the course of a ten year span with different reasons behind the purchase. The Frankensystem was put together to turn on some equipment that wasn't being used in an attempt to prolong its lifespan.
Disclaimer: Since I have no way of testing the different combinations other than my two ears - the above is entirely subjective.