Concerning the weight impact on the tone, I guess it has something to do with the response of your playing, how fast the notes articulate. A heavier bass should take slightly longer to make the note sound than a much lighter bass.
Do you state that lower weight supports a faster attack ? If yes, I'd be not sure about this, Jo !
Look at the heavy Fender boat anchors produced at the end of the 70s. They are famous for their ultra fast attack and the resulting fat slap tone.... Ebony is a quite heavy wood and the boards are chosen for their support to get snap in the highs. Northern ash as a body wood is the same.
Not only that weight is not identical with density (which is more important here IMO). The wood in a guitar absorbes certain frequencies produced by the waving string, and resonants some of its own tonal characteristics back to the string - and the physical attributes of a particular piece of body wood will determine, which ones. Of course only in conjunction with all other parts. My personal feeling is that weight alone is of minor influence here...