Do you want a DAC card that mounts on the Pi (e.g. HiFiBerry DAC+ or IQaudIO Pi-DAC+) or do you want an external DAC to go between the Pi and the amplifier? If the former there are a few choices like the two I mention. If the latter you should get the HiFiBerry Digi+ to mount on the Pi and you then have to decide which external DAC to connect to it by coaxial or optical digital cable.
The simplest and cheapest option would be e.g. HiFiBerry DAC+ or IQaudIO Pi-DAC+.