You should also note that I HIGHLY disagree with that review, given that I also hear all three of them (O2, C421-2227 and C421-8620). In a way, I personally find that review very misleading to many that have never had the chance to listen to all of three but need to base their buying solely on such opinion.
Here are some of what I have posted in the C421 thread:
I also personally think that trying to driver hard-to-drive headphone with portable amp is mostly a waste of effort. Sure, in some case you can expect really good SQ when driving hard-to-drive headphone with portable amp. C421 probably will do a very admirable job as well. However, it will not likely going to beat a desktop amp simply because of the limitation of power and PCB space for the circuit. If you have spent that much money on a full size headphone, I don't see the point of not trying to get the best out of it. Personally, I'll invest in a good desktop amp and treat portable amp as more of a compromise for traveling.
Thank you so much for your detailed comments!
What you said about portable amps is certainly true, maybe I'm expecting too much. I am not commuting much these days and at home using mostly full sized cans, at office yes.. at most the HTX7. I cannot use an open back here :S.
My idea is to use the amp to hook up to the notebook (and Dacport LX) and maybe for some bed listening with my Galaxy S. For now I guess that C421 is the best option right? I don't see anything smaller as good and at a good price.
As you said as the O2 is similar to the C421, I will later when I upgrade my cans, buy a full size amp.
With the price of the C421 being 270% more than the Cmoy, what % of more sound quality should I expect?