I first saw Mr Marvin's Cable Tailpiece at the 2007 ISB convention and when I got home I measured my Bass and ordered one.
It only took a half hour to install, anyone could do it, and the difference it made was worth far more than the $80 -$90 he's asking for them.
The resonance and depth of sound added were amazing, and a wolf tone was tamed as well.
Also, this tailpiece gave my Bass a much smoother sound and maybe a little more volume and projection.
It made such a difference that I ordered another tailpiece and installed it on my other Bass with similar results.
After hearing the difference in my bass, two other local Bassists have also made the switch and several more are interested but haven't got around to it yet.
I'm convinced, and for the price I don't think you'll find a better upgrade for yor Bass.