They're not driving anything except your headphones. The 305's have their own built in class D amplification. The RCA outs on the DAC come in 2 sets. One goes to the headphone amp the other goes to the LSR305's. get yourself a dual RCA to dual mono TRS 6.3mm cable for cheap from the rainforest. Sorted.
You won't beat the 305's in their price range. Horn loaded tweeter too. Awesome speakers.
Before I pull the trigger, can I go dual RCA to dual XLR also ? I presume it makes no difference?