Zabiaud, Rowland newest M625 clas a/b amp, as well as ICEpower-based amps in the 300 series like M312, M301, and the older M302 have a special current rectification system based on a technology called active Power Factor Rectification (PFC). This gives them a very significant amount of advantage in overall musical subtlety of the output, compared to M501 and M201. There exists an external PFC-based rectifier by Rowland called PC-1 that can be applied to M501 and M201.... and should bring those amps closer to the performance of the higher end ICEpower based 300 series.
Model 7 is a 25 year old class a/b design..... It is very sweet and has an enormous power reserve, but has neither the transparency nor the musical resolution of newer Rowland amplifiers.
Hope this helps.
Guido