If I may Blair, I would like to add a couple of more.
1) The component must be tube rectified. What can happen is that the rectifier tube can soften the sound too much.
2) The component must have choke(s) in the power supply. In this case, the power supply becomes more reactive when we want the power supply to be as linear as is possible.
As Blair mentions, the design is very important.
Cheers.