I emailed the company and the rep stated that the bass model is an altered version of the Stealth Pro model. One simply specifies on the order form which one is desired. The dimensions are similar to a Strat pickup, but there's a new one for 7-string guitars that would work better for bass string spreads, I think. The electronics and battery harness are what is altered for bass application. The Stealth C model mounts to the headstock and vibrates the whole instrument. The advantage is that there is no routing necessary, but it may not give best results. I thought it might work for vertical basses, due to the lack of damping from the player's body. Also, the added weight on the head isn't really an issue
I should add that I've only played a Strat with one of these in place, but was able to approximate a cello or violin being bowed. It seems a bass would be a natural fit. I think a cheap prototype could be made with an SX bass mounted to an instrument stand, then if it works you can perform surgery on your Barkers. Unfortunately, I've experimented enough this year. Gotta keep mama happy!