Any idea how to: * Know more or less for sure it's time for a second batch? * Differentiate this second batch from the first one?

Same defect here, in freshly bought units: * First unit had noticeably more white noise in the right cup from the very start. Returned. * Second unit had noticeably more sound body in the right cup (like, literally, voices in the podcast were coming more from the right cup, also, bass had more...