I'm currently using my Asgard 2 as my main headphone amp (for HD650, UE700's and Momentums), the source being a Grant Fidelity Tubedac-11. I'm using the tube output into the Asgard, and using the preamp outputs to connect to my Emotiva Airmotiv 5's. The dac is exceptionally quiet, so no noticeable added noise into the asgard. It works just fine as a preamp, but has a "hardening effect" on the tube-based signal compared to the dac's built in headphone amp (op-amp based) or the tube out directly to the speakers. The GF dac amp section has a certain midrange richness that is very desirable, yet is lacking power compared to the A2. Make no mistake, the A2 is a terrific value and a great amp for the price, I just feel it has a slightly "hard" or "clinical" sound to it when compared directly to the dac's output. I should mention, I'm running the Tubedac with a JJtubes E88CC tube in the buffer stage.
To my ears, the 650's are a little less warm when heard through the A2. My Crack is on the way, so that should be a fun comparison. In low gain mode, my UE700's fare a little better. Because the UE's have such inherent midrange richness, the A2 doesn't have much of an effect there. Very Smooth. The Momentums are very easily powered by both the Tubedac and the A2, and show very audible differences when A-B'ing between them. With the volume closely matched between them, I played a variety of music including the new album Incitare by Volto! (Fusion band with drummer Danny Carey of Tool) which has serious low bass punch, Moving Pictures by Rush, The Empire Strikes Back (1993 box set mastering), as well as a number of female vocalists such as Taylor Dayne, Tina Turner, and Andrea Corr (from the Corrs In Blue Album). With each listen, it was clear that vocals, guitars, string and wind instruments had a bit more realism and life through the Tubedac than from the A2. That being said, the A2 has far more power and dynamic punch than the Tubedac headphone out, not to mention slightly better low-end response.
All in all, the A2 is a highly competent performer that is a incredible value. I find it to be very listenable and versatile, just possessing a slightly clinical edge where I prefer a warmer presentation.