The issue is not what is the output impedance; whether its tube or transistor. The issue is that the output impedance can vary with frequency while not at the same time having flat frequency response into a certain load impedance. IOW the output impedance curve is not the same as the frequency response!
I've seen this phenomena with solid state CD players and tuners- its really a function of that output coupling cap. You can't always make it as big as you want; the cap construction itself often restricts what is the largest expedient value to use for best sound. This is particularly true of film caps; electrolytic coupling caps have a bad reputation but they do allow for much higher values.
We avoid the issue altogether in out preamps by using a direct-coupled output we patented. In such as case the output impedance curve looks exactly like the frequency response curve.
I've seen this phenomena with solid state CD players and tuners- its really a function of that output coupling cap. You can't always make it as big as you want; the cap construction itself often restricts what is the largest expedient value to use for best sound. This is particularly true of film caps; electrolytic coupling caps have a bad reputation but they do allow for much higher values.
We avoid the issue altogether in out preamps by using a direct-coupled output we patented. In such as case the output impedance curve looks exactly like the frequency response curve.