Hey I have never personally used Diesel Amps but there is a pretty long standing Shop over here in Pinellas that has been using Diesel for quite a few years as there entry level amps (Radioactive / Custom Edge Audio) and the owner never once had anything bad to say about the line that I heard.

Now as far as used amps unless you are buying it from either a reputable shop that took it out of the persons car themselves and tested it while still in the car or a reputable person then I would not recomend it because like MCOR said there is probably a reason why it is no longer in the persons car.