I have both in house right now; I was one of the 1st dp-777 owners in the US and continue to love it. However, I really wanted the cability of playing DSD files natively and just don't have the space (and don't want) two DACs. I have had the MA-1 for around 3 weeks now--I was pretty worried that I would sorely miss the dp-777's sweet sound when I removed it and it's tubed output stage from the system.
IMHO, the MA-1 and the dp-777 are more similar than different on redbook and non DSD high res. The MA-1 might have a tiny bit less 'romantic' midrange (I love the dp-777's mids), but honestly it's very close. The MA-1 has a bit more detail and soundstages a bit better in my setup. I could be very very happy with either dac; again I think they sound a lot alike.
But, the MA-1 playing DSD files has me over the top happy--I can't stop listening to DSD!!!
Bottom line, again IMHO and in my setup, if DSD isn't important to you, I would stick with the AMR and spend the upgrade funds elsewhere. If you want to do DSD, grab the Meitner--I don't think you will miss the AMR.
I'll likely list my dp-777 for sale in a couple of weeks, but I'm not letting it go until I'm absolutely certain that the MA-1 fills all my needs (but I'd say I'm 98% there at this point).
Hope this is helpful in some way.