2x56: Prima Donnas:

High-quality, matched pairs of vintage 56 mesh plate tubes are increasingly rare and expensive.

Use tube dampers or high-mass isolation platforms to combat microphonics. Prima Donnas: 2x56

To understand the 2x56 configuration, you have to understand the Type 56 vacuum tube. Introduced in the early 1930s, the 56 was a super-control indirectly heated triode. High-quality, matched pairs of vintage 56 mesh plate

These tubes are notoriously "microphonic." If you tap the chassis of the amplifier—or even walk too heavily across a hardwood floor—you might hear a "ping" through your speakers. Prima Donnas: 2x56