I have an XM4, which shares some features with the XM6. It has variable crossfeed and bass boost. The XM6 will also boost treble. These features were fun to play with at first, but I never ever use them. I'm a bit of a purist and perfer not to add any equalization to the signal. The build quality is top notch. I use my amp when I'm going to sleep and it has a feature where you can set it to shut off automatically after a certain number of hours. I find this feature very useful. The XM6 has this too.
No matter what comments are made here, nothing will compare with your personal experience with the D4. I would say if you were happy with it, get another one. Especially if you are looking for a "Wow", you are probably setting yourself up for dissapointment.
However I know it is the nature of this hobby to want to branch out and get something better - been there done that. :-).