What does the rest of your system consist of? With most receivers or a low end amp/pre-amp and sub-par speakers, you probably won't be able to tell a difference. With a higher end amp and speakers, you may be able to tell a difference on some source material. I used to use a a separate DAC, but it is up in the attic with my pre-amps and amps, I got away from using separates and my higher end equipment when the kids came along a few years ago. My hearing isn't what is used to be either, growing old sucks.
Your best bet is to find a good high end audio store (we have a couple withing 30 miles of where I live) that will let you listen to several different setups with and without external DAC's, ideally with the same or similar equipment you'll be using, but even if it is different equipment, you should still be able to get an idea of what adding an external DAC can do for a system.
