I want to share my experience regarding headphone amps and DACs and why I don't believe in "scaling" at all.
In the last year I finally moved to some very good headphones: HD650, K701, HD25-1ii and UE TF-10 and bought the Fiio E9, E6 and the CA DacMagic Plus DAC/amp.
From what I hear the only difference when I use a headphone amp is volume, which wasn't enough for classical sometimes. HD650 for me sounds the same as long as the volume is the same, no matter if it is run from a PC (with or without the E9/E6), the DacMagic or my Galaxy S. There are small differences for TF-10, but only because of impedance mismatch, not some "magic" amplifier influence as you are driven to believe from posts on Head-Fi.
DACs have an even smaller influence. In a way I regret buying the DacMagic, I don't think I could hear a difference in blind testing with the Galaxy S. The only real difference is noise at high volumes: the DacMagic has none. And I don't want to hear "the DacMagic is only an entry-level DAC". It measures as good as the Benchmark DAC1; reading Head-Fi you are lead to believe the differences between an on-board DAC/phone and a dedicated one would be huge, but they are very, very small. Maybe 10 years ago there was a noticeable difference, but nowadays there is practically none for well-engineered DACs and phones.
Cables are one of the biggest scams in this industry: zero scientific evidence, and zero difference in sound. Out of curiosity I swapped the HD25 cable with the one from HD650, and I couldn't hear at all the "big" difference people were claiming.
Now people will come and say I have hearing problems or I don't pay attention to the details in music. I hear differences between all headphones, but almost none between the electronics I have. Even between the headphones the differences are not that big, when you reach a certain quality level it's normal for the sound to be more or less the same.
For me the journey in the headphone world is over for the next 10 years at least, I'm very satisfied with what I have now. I had a revelation when I bought a pair of B&W DM601 S2 speakers last month. None of the headphones I have can reach the instrument separation, soundstage and clarity these speakers have, and they were cheaper than the HD650 (both bought second-hand).
My advice for those interested in quality sound: invest the most in headphones/speakers, buy an amplifier only if not loud enough or there is a serious impedance mismatch, don't buy a DAC unless you hear obvious problems (noise, for example).