I've noticed that the later version of the Concert Bass series (red knob - post 1975) is basically the the Concert Slave Power Amp design. You'll notice that the later series doesn't have the distortion feature, uses only one rectifier (different power transformer), slightly different PA and preamp boards, etc.
Note sure where Sunn gets the 200W rating for the later series since the driver and output stages are almost identical to the earlier design. I have to assume that this rating is at 4 ohms since this design will over heat with any load under 4 ohms.
Also note that the later series has the handle on the side (ala Acoustic) and rubber feet on the side of the cabinet (now a four sided box).
The later design also appeared to come with a lifetime warrenty (again ala Acoustic).
