Headway EDM-1 and EDB-2 acoustic instrument preampliers known as the swiss army knife for the gigging musician has had a really good, in depth review in Bass Gear Magazine issue #15. It is worth checking out. If you dont subscribe to their magazine they have Open Flipbook on line. Pg 36 - 47. http://www.bassgearmag.com/bgm/Magazine
You can also check them out on our site here: Preamps