The 2000S with tube GZ34 rectifier tubes was rated at 120 watts RMS, and it was actually a bit more than that...more like 140 watts. If you replace the tube rectifiers with solid state diodes, then the amp is rated at 150 watts RMS, but again, its a bit more than that...more like 170 watts or so. The solid state rectifiers are more efficient at converting the AC to DC and don't lose as much voltage from the B+ when they do this, so you end up with more voltage at the tubes, which means more power. They went to solid state rectifiers instead of the tubes for a number of reasons......less cost, more reliability, more power, less heat. Even when they had the 2000S model that came with solid state rectifiers, it still came equipped with tube sockets for the GZ34's for awhile.
The sound is slightly different between the two models, as is the feel. The solid state one will have a slightly quicker bass response, and the highs will be just a tad more present due to the higher voltages in the amp. It can be modified either way though so its not a big deal.
Greg