You should never connect speakers with a headphone amp! use a speaker amp!
Line-in: this is used for connecting source to amp in the FUN. you should only connect speaker here if you want to use them as a microphone! (so NO!), This connection is missing in the NFB-12.
Line-out: this is used for preamp, connection to a power-amp. So if you turn the volume-knob the signal will change. this is the same as the DAC-out the NFB-12 to variable output-level signal.
DAC-out: this is used to send out a fixed output-level signal, same as the NFB-12 DAC-out set to fixed output-level. used to connect to a normal amp.
None of this will work directly to passive speakers, only to active speakers!
And if you really need to connect them to passive speaker, you should use connect them where you connect your headphone. The result can be sound very bad!
You can change between fixed/variable with a jumper in the NFB-12, while in FUN you have two different outputs.