There are no major differences between the early and mid models of the concert.
Concerning the power amp, the primary differences in the later models vs the earlier models is an increase in emitter resistance from .27 ohms to .33 ohms, and the addition of a diode/resistor network between the output transistor base and speaker output/emitter (this would limit maximum signal input to the output transistors).
The later models are rated at 200 watts, probably achieved by a 2 ohm load. Concerts prior to that are rated at 150 watts into a 4 ohm load.
For what it's worth, I would never drive a 4 transistor output stage into 2 ohms, especially 2N3055s which is what both early and late models use. Most transistor power amps capable of "easily" driving a 2 ohm load will have at least 6 if not 8 output transistors capable of handling the increased current.
It is simply not worth the extra 50 watts, which you probably won't even notice... to the life of the output stage.
But that's just me. I would rather have an amp that will survive easily at 150 watts over taking a chance at 200 watts and failure.
There are other problems with running at 2 ohms, but I am too lazy to list them now. Your mileage may vary.
