This is due to the impedance curve: not lower than 7 ohms. It plays down to 40 Hz without any loss of sound pressure. Between 1 and 3 kHz, the speaker will play 2 dB louder. With an amplifier, the sound is livelier without feedback.
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You can see all of this from an impdance curve without listening to him.
Works good with set amp,but I would not combine high-resolution analytical sounding tubes with it, it tilts the sound image into the unpleasant.e.g newer siemens or ribbed plate tfk.