R2R-11 works well with the HE-400i (I sometimes use this combo myself). The DAC is better than the amp, and can also drive an external power amp via variable pre-outs (you need to move a jumper inside the case). Then you can use a good second hand power amp (Quad 306, something from NAD or Rotel) or one of the better modern Class D amps (e.g. Temple Audio) to drive the HE-6.