Went to the Woodstock Luthier's show this past weekend. Spent a half hour with famous luthier Linda Manzer, who amongst other things had a tonewood amp sitting on the back of one of her guitars--a 1977 effort that had been fitted by the owner with a preamp and which somehow wound up back in Manzer's hands . She had seen the Tonewood Amp in action and decided to spread the word--it's produced by someone whose work output she regards highly. Got to play with it--it can only be used on a guitar with preinstalled preamp and pickup jack. It boosts volume a bit as well as providing effects; primarily works by vibrating the back of the guitar. The installation is completely reversible, and the only conceivable damage to the instrument would be tape adhesive adhering to the inside of the back of the guitar.
It has an output port so it can be plugged in, but its primary joy is in a purely acoustic setting.
My wife bought one for me as a holiday gift, so I will post once it is in my hot little hands. I will be using it on my 2005 614CE.