I have the DP-U50 and had the M-Audio Revolution 7.1
Unfortunately, I can't really tell you what the analog out on the revo is like though, since I never ran headphones directly out. My impression of the DP-U50 out though...
When using it as a DAC, it really really sounds sterile and unengaging. It's not extremely detailed, but just extremely boring. However, using analog from the revo 7.1 really livened things up. It was as if the music was brought back to life. Also, I found the treble on the DP-U50 using USB a bit harsh. Not sure why that was, but it can get a bit annoying.
The DP-U50 makes an excellent input selector and I highly recommend it. Another nice thing you can do is connect it via USB and while using foobar kernel streaming (effectively cutting out other sounds), have the USB audio signal mix in with your digital. That means that your revo/emu/whatever could be outputting with kernel streaming, and you could set the USB audio to default and hear everything inside windows as well. Very convenient.
I feel that the DP-U50 is a worthy purchase and the out will satisfy you for a while. But save up for an E-Mu0404, as it's much better than the revo 5.1/7.1
Pairing the two together with an amp is great.