There are a number of things that can be considered when using a tube as a
mixer. I always thought the 6BE6 was more prone to microphonics than other
tubes. That does not show up in the tube manuals.

It is certainly possible that the 6BY6 may handle higher signal levels as a
mixer, even though not designed as one. Probably the best test is with a
signal generator and in the actual radio.

Colin K7FM