The Douk Audio P6 Mini is a small stereo preamplifier with tubes, USB and Bluetooth, all at a very affordable price.

Recognizing the nonlinear behavior of the 12AX7 vacuum tube, used in audio and old op-amp applications, Dr. Dyer compares various models for fit to real-world device data.

It's old, British and cool, and you might even be able to pick up one for 500 bucks. But with its edgy Vox, underpowered ...