| Test | 12FK6 12V 33 | 12FK6 13.5V 33 | 12FK6 15V 33 | 12FK6 16.5V 33 |
| Frequency response (from 40 Hz to 15 kHz), dB: | +0.03, -0.12 | +0.05, -0.08 | +0.04, -0.10 | +0.04, -0.09 |
| Noise level, dB (A): | -97.6 | -97.8 | -98.0 | -97.9 |
| Dynamic range, dB (A): | 94.9 | 94.6 | 94.5 | 94.5 |
| THD, %: | 0.536 | 0.956 | 1.194 | 1.121 |
| IMD + Noise, %: | 0.476 | 0.987 | 1.132 | 1.064 |
| Stereo crosstalk, dB: | -68.7 | -68.9 | -69.0 | -69.3 |