The M7 has more detail, focus and speed but less bass and perhaps a smaller soundstage than the Ref7.1.
The Ref7.1 very clearly had more bass and a much larger soundstage than the PWD. But to be honest, it was the neutrality that I felt was most important. The PWD MK2 got a little shouty with the prominent, liquid mids at higher volumes with some songs. The Ref7.1 just sounded like music all the time. I forgot about the DAC, whereas I always knew with the PWD I was listening to a synthetically reproduced version of the music. It was great for some tracks that benefited from the mid-representation, but it took away from others. Neutrality is key for me. I like to know what the recording actually sounds like, without it being analytical and unforgiving. All these DACs are fairly forgiving, but the PWD adds a bit too much of its own flavour to everything IME.
Edit: To add some more info; the OR5 (with all upgrades) made the Ref7.1 sound a lot better than the PWD MK2. In terms of detail and layering + imaging they were both pretty close then, but in all other areas the Ref was better. The level of improvement I got from the OR5 on the Ref was remarkable, but it didn't seem to improve the PWD as much.
