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As a point of historical fact

One of the champions, in fact perhaps the leading champion of wide bandwidth audio power amplifers in the early to mid 1960s during the transition from tubes to transistors was Harman Kardon. Amplifiers such as A500 using self biasing tetrodes 7355s had a bandwidth of around 50 khz. One of their first solid state designs A1000 had an advertized bandwidth of 1 Mhz and actually tested out to about 1.5 Mhz. These attributes are of course rediculous for audio use and in my experience having lived on the infield of a commercial FM transmitter, extended bandwidth creates more problems than it solves by making these circuits far more susceptable to induced rf noise unless they are very well shielded. This is especially true in the high gain phono section of a preamp. Dynaco Stereo 120 type amplifiers inserted a series inductor as a coil wound around the output DC blocking capacitor to limit bandwidth to about 100khz.

I agree with tom about the output transformer being the main source of audible distortion, especially frequency response distortion in vacuum tube amplifiers. The main culprits seem to be eddy current and hysterisis losses in the core. B-H curves even for supermalloy are less than ideal probably orders of magnitude worse than the nonlinearities of either most tubes or transistors.

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