As far as I know, the only difference between Modi's 1 and 2 is the switch, which allows drivers to bump up the USB performance to 24/192 (without dirvers, both are plug-n-play via USB up to 24/96). The Uber Modi might sound a bit better via USB because it is AC powered, but I wouldn't try to sell one based on that. The Uber models biggest appeal is having multiple inputs, while all other Modi's are restricted to USB OR optical.
As for Magni vs. Vali, Vali has tubes, and HD600 owners tend to prefer tube amps. That being said, I still like my Magni with my 600's. Vali would be a good choice if you wanted a taste of tubes without spending $350 for Valhalla (a "real" tube amp), but Magni might be the only amp you ever need, and it can power just about anything.
Lastly, skip Loki. Unless you know what DSD is...AND you are interested in DSD...AND you already have music in the format.