Tube Buffer Question

Musical Fidelity sold Tube Buffers with and without separate power supplies. Can anyone explain the differences/benefits of using the tube buffer with an separate power supply?
I used a MF tube buffer for a period of time, both with and without the external power supply. The addition of the power supply had two results in my system: Lower noise floor and improved dynamics. The differences were subtle.
I have one of those if you are looking for one!!!