I ask because I came across one in a local shop for £525, and was trying to find out anything I can about them. The guy in the shop had no idea what the 9V cavity was for until ai figured out it was for the boost.

I think they’re quite rare, and for that price I surely think it’s worth it! As far as I’m aware the TBX is a mid boost, which I think is 25 db. I didn’t know much about it either, so I had to ask my mate who is more in the know.