It'll be more than enough for HE400s. Spend any more and it's just nuances and preferences in colouration. As for amps though, HE400s are very forgiving (unlike other orthos or in fact many dynamics) yet scale very well too, so you'd be happy now and in the future when you invest in a bigger upstream system.
The JDS ODAC has the option at $10 to have RCA outputs (vs only 3.5mm)... guess thats a good idea? Next purchase will be a Desktop Amp (Probably Asgard/Lyr) - and they seem to mostly have RCA inputs... I know i can get cables from 3.5mm to RCA, but surely RCA to RCA would be best? You've been a great help.