cones under preamp

Hi folks, whenever I put cones (ebony wood or aluminium) under my preamp, there is an audible change to the sound. Whether this change is an improvement or not, I'm not sure. Although this tweak adds more detail (and probably also a tad more spaciousness) to the sound, I also hear a bit of thinning out of the lower mids and a shift towards the upper mids and bass. Soundstage is pushed further back. The bass becomes more prominent, but sounds as if it is separated from the rest. It's as if there is a "hole" in the midrange and the sound loses some immediacy. This phenomenon can also be heard when I put the cones under my CD-transport or DAC. Is this a well known phenomenon with cones in general?