Thinking about getting an after market amp

If you think the HD650 runs fine off the D1 then don't allow curiosity to command what your wallet does. The shortcomings of a system aren't from the component count or total cost, but from what you perceive is lacking in the sound. Best that you listen to other systems, and not necessarily just headphone systems. Speaker-based systems might be available in audiophile shops near you, or in some car audio shop's demo cars (not all car audio is about obnoxiously showing off how you can blow the windows off).

Just an example, I didn't just start off going for audio equipment, but I've always had Walkmans and Discmans since I was 5 years old. Then later I began complaining, "What the hell is James Hetfield still strumming that guitar when I only hear it on the intro? And why do they need Jason Newstead, I never hear his bass anyway." Later on I had a band, saw a lot of live performances, and eventually heard a car built for IASCA. Then other people got me to go into the audio shops that I used to think had "snooty" proprietors, but apparently there was only this one shop owner that people either hated or liked. I modded my car, eventually set-up a small system in my dorm, then got frustrated with home speakers altogether due to room modes and switched to cans. Throughout all that I noticed other things in the playback quality, like fast drums sometimes losing some notes or just grab your attention more in other systems. Our drummer before is just getting into getting portable audio gear, and I brought him to a meet so I wouldn't be speaking in abstractions about how the gear can make the music sound more "alive."

If you'd really want to try out if there's a difference, both were designed with the 300ohm Senns in mind, but I'd bet on the Asgard (partly for the RCA input). Do take note it's a fairly transparent, low-power Class A. Meaning it will most likely transmit the sound off your D1 without changing the basic response. That said, the D1's headphone out might sound different through the headphone out and the analog outputs, although I'm not sure if the tube only goes out the variable lineout. In any case, there is a chance you might not immediately hear the difference, but the thing with low-power Class A's is how smooth they are without the tendency of tubes to get too thick in the midrange. Make sure you vent it, and don't stack it with teh D1 either, since the whole chassis acts as the heatsink.

You can, but I'm just the sort who'd avoid converters as much as possible. I did only say "partly." The rest of it is because the Asgard will save you $100.Just be sure that you buy a calbe that's already RCA>3.5mm, since the male 3.5mm>female RCA plugs might stress the jack if you connect heavy RCA plugs on it.