After buying the Sennheiser 6XX's, I'm noticing the shortcomings in the Dragonfly Red, even with the Jitterbug decrapifier. It gives a hint of high-end, but falls short in a number of ways. The highs are bright and hard, and detail is missing (maybe partly due to the cans though). Piano solos are an incoherent watery mess.
I think JJayJJ is right, Drop hasn't offered any really good DAC-Amp combos since I joined. They all have a significant compromise somewhere, either in the DAC or the amp section or in build quality.
Update: After more break-in, the HD 6XX's have blossomed and the Dragonfly Red combo is blowing me away. Please ignore most of the DAC shortcomings I mentioned above. The highs have become much more liquid and natural, and I'm getting huge amounts of masterfully resolved detailed that I never thought possible from this DAC or the headphones. Piano has become more coherent.
The only remaining weakness, and maybe this will also change with more break-in, is that piano notes remain tonally fuzzy and fail to image -- it's like listening under water. When acoustic piano is recorded well and reproduced correctly, you should at least sometimes hear it as a percussion instrument, with clear attacks of the hammers on the strings producing well-delineated crystalline notes with metallic overtones. Since I'm not hearing problems on other instruments, it might partly be due to miking technique in the studio, but otherwise I'm guessing it's the DAC.