You can pay an absolute fortune for a good headphone amp but much of this money goes into snake oil. A decent DAC chip is not expensive and amplification is not rocket science. SMSL is a Chinese company that's making waves with good quality audio gear that means you don't have to break the bank to get good sound.
This DAC/Amp receives optical/coax digital PCM and sends it out of the headphone output at the front and via RCA line-out at the rear. The volume controls the headphone only - as it should - leaving the line-out to be a fixed volume. I've got this DAC set up on my desk where I feed it an optical signal from my motherboard, the line-out then goes to a hi-fi amp and speakers. The sound quality is excellent both in the phones and via the line-out. I've used the line-out to feed an SMSL SA-50 amp and a Marantz MR-603 hifi and in both cases the results were superb.
Be aware like most headphone amps there is a little channel imbalance through headphones at the quiet end of the volume taper but this is gone at normal listening volumes. Also as is often the case with excellent but inexpensive Chinese audio gear the power supply lets the side down with cheapness and potential electric death. My 793ii came with a two-prong switching power supply and a travel adapter. While the power supply seems fine - I didn't like using it with the travel adapter and bought a 9v switching power supply from Maplin with a proper UK plug.