There is no way to bypass the internal amplifier on the Eris 3.5BT speakers.
on the back of the UDA-1 you'll see where it says "D/A Line Out" using the RCA Red and White connectors connect to the RCA Red and White connectors on the Eris Monitors, then the amp no the Eris will be used and the amp in the UDA-1 will be bypassed.
If you want to use the Amp in the UDA-1, you'll need to connect another company's passive speakers to the speaker outs on the UDA-1.