Don’t know of any credible studies. I do know for a fact that DC blockers can work to reduce DC-offset which is a common power quality problem. Some power providers are willing to conduct a quality audit if you have legitimate concerns, like wonky appliance behavior or buzzing transformers. That would at least give you an idea of whether power conditioning is worth pursuing. I find that if the gear is well-designed, the quality of the mains power is not as critical. A high quality power supply will largely filter out the incoming garbage.
You definitely want to stay away from amps with cheap toroids though it can be difficult to identify which brands are using quality ones.
You definitely want to stay away from amps with cheap toroids though it can be difficult to identify which brands are using quality ones.