I have the GX and they are great headphones, can be driven by anything. To answer your question on how you should drive them, let your ears decide. If you can't tell the difference then just go with whatever is the most convenient.

The cable without the microphone won't be any different in sound quality, but just a nicer quality cable in general and nice and long so if you are going to plug them into a dedicated amp and then set back on a sofa or comfy chair, a nice long cable is great.