Well, this has become a lively discussion. I can see very well where @Torq is coming from. He has a well trained ear, and the opportunity to compare with more equipment than most of us. And I’ll explain some of the whys for us having what are actually similar opinions but differing reasoning.
First, I have had my HD-580s for at least 20 years, and the HD-6xx’s for about 4 months. Also in the last 4 months I have been listening more to other headphones, a couple of which I put through very critical listening to write up here, and to my old STAX, because I got a gorgeous tube headphone amp to run them on. As I travel frequently to the house my wife inherited, I keep the 580’s there now, and just wanted a good Senn for my usual home listening.
Second, I love the Dragonfly products. I have an iPhone 6+ as my everyday device, and with either camera adapter, I am able to have a compact, pocketable system that is far better with a range of headphones that I would otherwise have. Portability and convenience is a consideration. I use this system with entry level Grados (Which I also like, but refrained from suggesting because they are a love em or hate em headphone), The Senns, my Koss “Weirdphones”, and even at low levels, the notoriously power-hungry HiFiman HE-560. On the latter, it’s really easy to show the power-based issues that @Torq mentions.
For non-critical listening, I am likely to power the HD-650s with my Headroom Standard amp (the new one, not the original, there is a big difference). It often sits right next to the recliner - Did I mention I like convenience? I find that the DFB can adequately power the Headroom amp inputs - and so can the iPhone 6+. If the source (CRITICAL POINT HERE the source rules - if it’s not good, nothing is, worse than in the speaker world) is improved with the DFB then I find I get better sound using it along with this particular amp. I’m talking about a source that might need a DAC for higher resolution - I can use an android phablet instead of the iPhone and get high quality FLAC and MQA, for example.
@Torq has a much wider selection of amps than I do, and can speak to the woes of using other amps with the DFB and DFR. I can only speak to the use of this particular amp, which matches reasonably well.
That said, the DFB is most certainly NOT my preferred DAC with the HD-650/6xx. I far prefer using my iFi xDSD, which has considerably more power, and allows me to use its “dual mono” mode, which while not being “balanced” is a lot like running two monoblocks in your regular hi-fi rather than a single stereo amp.
But the biggest difference in our reasoning had to do with budget, what you already own, the path you will take, and how you may listen. Both of us made some differing assumptions. I focused on your prior musical listening and the comment that you had not spent time with many enthusiast loudspeakers and hi-fi systems, but more professional guitar sound. I also saw your characterization of yourself as a “newbie”. I found that in my wallet-bound journey, I would slowly educate my ear, use what I had until I knew what was defective or annoying, and make a careful jump at some point.
Heck, I didn’t even investigate DACs until their prices came into affordable ranges, and it was then with some skepticism. I was more focused on good amplification. I never had an 8-Track phase, much of a Cassette phase, or an MP3 phase. I knew crap source when I heard it. I listened to records and CDs.
In the car, I did MP3, but the car ain’t a critical listening location. I’m not pure - I do have some MP-3, but only when that was the only available format for the MUSIC I wanted.
Finally, I used to work in an industry where new users - newbies - were gold. You can only get good initial evaluations from a new user. I’m sure @Torq has experienced this also. So all of us will be very interested, no matter how you do it, in hearing your experiences with the exploring path.