Actually, distortion aside, class "A" single ended amps (tube or solid state) do not exibit crossover distortion. Push-Pull amps, especially solid state ones can have nasty garbage at the zero crossover point, which has to be taken care of with negative feedback, which has it's own problems. This is really only an issue with very efficient horn loaded speakers since they amplify any low level grundge upstream. The signal path has to be squeaky-clean.