I have W/P6 with Atma-Sphere MA1 MKII.2 and to me it is Audio Heaven. I tried Pass X350, VTL ST150, Kora Cosmos, Bryston 4BST as well as others. All of the above were great in their own right but was missing something. The Pass had the best bass. The Kora had the best mids. As for VTL I was trying to listen to the MB450s but my dealer couldn't get a pair. The ST150 sounded good but just not it. The Bryston made my head hurt :-(. I was hoping to listen to the 14B but my dealer had just sold it. I then listened to the Atma-Sphere and was blown away. To me the best overall amp I have ever heard but this is just me.
The Atma-Sphere mids and highs are simply unreal. Talk about a singer being in the room. The bass compared to a Pass, Bryston, Krell isn't on the same level although close. What makes the Atma-Sphere so good is they just sound right. They are the best amps I have heard from top to bottom.
I guess the real answer is get your hands on some amps, take them home and listen. Either way you go you will be happy.