I am real sorry about the hearing loss (and worry about young people and their headphones).
I fail to understand how a separate pre amplifier can improve the sound (other than by having tone controls). Pre amplifiers once were necessary to bring the various analogue sources to the same level. If you only use digital sources there is no longer any need for this. A DAC with variable output like various Benchmark models (or the Marantz HD DAC1 or Pioneer U-05 at a more modest price level) will do the job just fine. If your DAC does not have a variable output, add an outboard volume control like the TC Level Pilot, or just use the digital volume control in the computer or disc player like the Oppo 205.
I fail to understand how a separate pre amplifier can improve the sound (other than by having tone controls). Pre amplifiers once were necessary to bring the various analogue sources to the same level. If you only use digital sources there is no longer any need for this. A DAC with variable output like various Benchmark models (or the Marantz HD DAC1 or Pioneer U-05 at a more modest price level) will do the job just fine. If your DAC does not have a variable output, add an outboard volume control like the TC Level Pilot, or just use the digital volume control in the computer or disc player like the Oppo 205.