You should'nt need a "preamp" for digital signals, just connect your optical or co-ax cable from the digital output of the player to the digital input of the DAC and it should work.
You can't increase level by amplifing the digital datastream, you need to multiply the data words with a multiplication factor.
The info isn't in the level, it's in the code.
You need a DSP.
What is downstream of your DAC? Are you going directly into an amp? If it does not have gain controls, maybe a preamp between the DAC and amp will help.