I don`t have an AH 900 but I have a Denon AH-D5000 and started by buying the Parasound Z-Dac. The Z-dac is pretty decent and includes a fairly good headphone amp, my only gripe is the 1/8" headphone jack so if you have a 1/4" plug you will need an adapter. Inputs: coax - optical - usb. Outputs: single ended or balanced. US price $475,- which is quite inexpensive. In UK or Norway it costs about $800. You will find many good reviews on the net.
Regards from Norway.