What is the source and context of that statement?Hi, does it any truth 12at7 sound better drived by low impedance source,please?
A tube has characteristics but no sound of its own (except if you drop it ). The sound influence (meaning frequency response and non-linear behaviour) depends on the whole circuit including the tube. Without specifying the circuit no evaluation is possible. And "better sound" is a rather subjective assessment and depends on the function of the circuit.
The 12AT7 was primarily designed for lower impedance HF applications, while the 12AX7 being a more linear, higher gain type was optimized for audio purposes.