And again, forgive me if I'm coming off as a bit crass. But if you can be happy, content and even joyful, I don't get when the consuming anhedonia comes into play. The only thing I think I'm understanding is that you cannot alter it despite what compounds or activities you may partake in. You 'have' stated that, in high doses, amphetamine will do something. That's a step in the right (or wrong) direction. It indicates that some areas of your brain are desensitized where they shouldn't be.
Many years ago, I was put on an SSRI due to MDD facilitated by a family tragedy. It helped. Prior to this, I dabbled here and there with the rave scene and it's most popular drug taken during one. After SSRI use, and the resultant positive treatment response (yet ongoing SSRI use), I couldn't feel a damn thing from any reasonable amount of this rave drug. Nothing. Others are flying. Me, zilch. Environmental factors can play a role in structural changes to the brain and how it functions. Were you and your brother living together or in close proximity when this phenomenon became pronounced?