A bazillion people (ok, maybe not really) asked me about the Master 7 vs. the Hugo. Short answer: I think the Master 7 will need an OR5 to sound as clean and natural as the Hugo.
The only way I could A/B them easily was to use a Digital Interface with the I2S output going into the M7 and coax out into the Hugo and both going into the Studio Six. It made for easy volume matching.
The Master 7 had a slight hardness to the sound in comparison to the Hugo, which, like I described before, has a liquid flow to how it delivers music, but an infinitely fine liquid that can bring out micro detail and texture well. It's not a warmth like we describe other equipment, but a smoothness that comes from a lack of distortion. It is rather like a digital version of the Studio Six (or a good SET amp).
The Hugo is best from my AP1 via coax, but all the inputs are good, including the USB from my MacBook Air, with minor differences between each and depending on the source used.
The amp is pretty good, so far, with HD-800s, except when you turn the volume up loud where I feel it sounds a bit shouty. At quiet to moderate volumes it is lovely, however. It is a good example of why listening volume is very important and why people can end up with quite different impressions.