I've also read some people saying that the 650 are a big improvement over the 558's and that they require an amp. Can you explain why exactly you can't just use it out of the box without an amp? And what an amp does exactly? And what's the best type to get? And have you used one? And I think I may get the 650's for home use and the Momentum's for on-the-go usage when I'm out and about. Thanks!
I'm not the best to explain this, so if anyone sees something off please correct me.
The HD650s require more current to produce a "loud enough" signal. An amp takes a signal and amps (amplifies/boosts) it. Technically speaking a solid state amp is the best type of amp if you're looking for accurate representation of music, but some people like the sound of tube amps, there really is no right or wrong here. Yes I have used one, I have the Shiit Magni. If I were to recommend any amp to someone starting out, it would be this one. It's solid state, and allows for a wide range of different headphones to be used with it. ie. high impedance cans such as the HD 650, down to low impedance cans such as Denon AHD2000, ATHm50s, etc. The best part is that it's only $100 and comes with a 2 yr warranty.