Well, I was enjoying my O2, I gave it to a friend, but he said he couldn't get it to work.
On further review one of the 9V batteries looks somewhat deformed. Not sure if it was an internal battery issue or an amp-charging issue.
The amp lights up when plugged in, but no sound whatsoever. I thought it must be the MOSFET's. Replaced those, no dice. Only reading 12V between the battery terminals with an 18V AC supply line.