As far as I understand TA2020 could be just a TA2021B in a different package.
But I still wonder: do they sound different?
Had you the chance to listen at both of them?
Biulding an amp3 surely is challenging but I'm planning to use an external power supply and I don't need the extras of amp6. This drives me towards amp3 but, while I read that the TA2020 is probably the better sounding tripath, I'm not sure about the sound of the TA2021B.
PS: I'm posting that to amp6 forum too, I hope "crossposting" is not a problem.