The key thing to understand about Harmonix feet is, they are 'tuning feet'. They are 'absorbtion' type isolation devices instead of 'directional' devices designed to drain energy to ground, or another absorbtion device (like Walker valid points). I've had a play around with older Harmonix feet which added a touch of warmth/richness, however the newer feet such as this Anniversary model are more neutral. Harmonix feet sound a bit softer (more forgiving) compared to Stillpoints for example, but a set of (4) Ultra 6's + bases will set you back almost twice as much. The Harmonix feet are things of beauty. It is doubtful you would be disappointed, and likely would get a very nice improvement. Ultimately you'd have to try them under your equipment to see if you like the sound.
Just for fun, you could try a set of (3) or (4) Stillpoints Ultra Mini's which are quite a bit cheaper, but perform very well. Tonally, Pro Audio Bono (PAB) feet are somewhere in between Harmonix and Stillpoints, and seem to strike a nice balance. The PAB feet are also good value imho (as are thier isolation platforms).
