increasing impedance of headphones
You have several choices:

Get a chip that can drive 8 &Omega;.

Put a buffer between the D/A and the headphones.  This buffer must then be able to drive a 8 &Omega; load.  The D/A only sees the input impedance of the buffer.

Use headphones with 16 &Omega; or more impdance.

Put a 1.4:1 audio transformer between the D/A and the headphones.  The D/A output will need to be 1.4x larger than before, but will see a 16 &Omega; load when 8 &Omega; headphones are connect to the output of the transformer.