Can you plug a headphone into the jack that you are exporting to the Magni to ensure that it has a signal?
Are you connecting the shipped wall-wart power supply and not something else you have laying around?
Lastly, is that an on/off switch on the back?
The ODAC is another great option. I think JDS Labs has them for sale.
For your price point you could go with the ODAC AND O2 amp combo for under $350. If you go go with this setup ensure you select or inform them you want the USB option.