Which one... Biamped nuvista 300's or a single m3?

You should be able to hear additional detail at lower volumes. It's a personal decision as to whether it is worth the additional cost. It could be something that’s very cool!

Its a difficult but interesting question that I am not an expert on but I can suggest a few things. First are you happy with your current M3? Its important to ask this and sometimes its important to recognize when you are happy or you keep chasing the rainbow, spending a lot of money and perhaps missing the perfect match which you already had. I have an m3 and am very happy with it and its match to ribbons. If you are not happy identify what is missing and then you will be in a better position to judge whether or not the changes you make fix the problem. Thirdly, musical fidelity is bringing out a new 20th anniversary piece, the trivista and you might want to consider this if you are going to be spending more money, it has 350 watts/channel. You may be able to biamp with the m3(if I am using the term correctly). I thought I saw something in the manual, in which case you could you use the main power for base and add something to the top, either the nuvista 300 or the a3cr (I think thats the number). You might email musical fidelity and garner their opinion. I too have wondered about these options for the future and would love to know what you found out. The last thing is a question I have asked before which involves improving the connections between the amp and the power supply. This might be another consideration. Hope this is of some help.