Semi, The 501, and the 201 Rowlands are much the same amp, with the 501 suitable for heavier loads. According to several owner reports, the two versions sound similar.
The Rowland 300 series amps are of a different cloth. They are a good jump in sonics. Regrettably, they are expensive.
Interesting thing about those amps you mention (Classe, Plinius, VTL, Pass), some owners of these same amps have happily replaced them with H2O amps.
It's analog power supply sets the H2O apart from the Rowland 201, and 501.