The Cafe Walter is supposed to be a 'reference' grade headphone amp. It does not have the usual amplifier controls, because it is supposed to give an accurate representation of what an amplifier would hear before the signal even hits the preamp. So think of it as an 'amp' in the truest sense of the word, that takes the unprocessed signal from your bass and makes it louder.
Any other colouration that you experience thereafter will be a result of your headphones colouring it. I personally don't have a use for something like that, a Pandora would suffice for me, but I can definitely see where the Cafe Walter would have its uses, especially when A/B'ing basses in a controlled environment, a la Basstasters.
Some other bassists use it as a 'trainer' because its exceptionally uncoloured output does not mask some mistakes that can be less evident with heavy processing or using preamps with a very particular voice. Moreover, the high headroom and lack of compression help to make the bassist aware of and control attack and muting.
Am I right Cherns?