I recently obtained a JBL Professional Series power amplifier Model 6233. I was very excited to get this amp. I have an original pro ad for this unit. What I was wondering if anyone knows about this amp. The amp did not have the "shorting plugs" needed to run the unit in unbalanced mode, but I was lucky to find a pair of 9 pin plugs to make the "shorting plugs". It works great. There isn't much information on this unit; although the service manual is available on line, thank goodness. I am wondering if mine is a prototype, serial # 10648 or someone changed the front panel. There is no silkscreening on it, but all the lights are in the same place and front air intake is the same; accept missing the filter holder. I'll try to post photos of the unit and pictures from the ad. This amp I am sure has quite the history. - harrypj