6CA7-Z / KT88-Z or WE 6CA7Plus / WE KT88Plus – which one?

Answer to an inquiry from Germany – related to Unison Simply Italy:

6CA7-Z and KT88-Z were released over 10 years ago to the market as the very first high end upgrade tubes. But over years, the factory’s lack of quality screening has flooded market with mixed quality. We (Grant Fidelity) no longer carry them as the ‘selection’ cost is simply too high and performance varies too much.