Hi Mapman,
How It compares. Well I've never heard any of these compared in the same system. So my comments might not be fair...
Benchmark.... What I heard was fairly fast, transparent, but leaned toward dry... not dry, just leaning that way.
The Bel Canto, As close as I've heard to dead neutral... I would say the weakness was sound stage, detailed placement, width etc....
SST Ambrosia, Even though to my ears, it doesn't sound like tubes. I would say, take what you would consider being dead neutral in Tubes. That's how I would consider the Ambrosia. It just has a texture to it that I enjoyed. Its phono section was great and the tone controls work without causing harm to the sound stage.