I have no issue with the volume knob. From what I can tell the vast majority here that actually own an AK100 or RWAK100 don't have an issue with the volume knob.
You do not need an amp with the RWAK100(for IEMs). I don't know about the AK100 since I don't own that one. On the other hand having a really nice amp is something you can do to improve the quality even more.
Last night when I was just working at my desk I hooked up the RWAK100 to my MKIIIB and it sounded fantastic.