Wut, AD700 is inferior to HD558 in pretty much every way, they're not headphones in same league. No, AD700 is not an improvement to HD558's. Its the other way around, without any doubt. HD558's will give you much fuller and more lush / smooth / refined sound, much better bass, more precise soundstage and imaging, etc. But it DOES need an amp to sound at its best. Its not really a 50 ohm headphones....since the spike around bass goes above 300 Ohms, so if you don't use a good source, they might feel weak and uncontrolled in bass department. An amp like Fiio E9, or even better the E17 would do wonders with it.
Ya, I'm referring is HD558 big improvement from AD700. Didn't realise I had typed wrongly. But thanks for your recommendation.