Placing the 1:4 in front of the autoformer would increase the source impedance to 6.2K which would then be reduced by whatever attenuation is needed. This added 12dB of gain would be offset by anything over 12dB of attenuation adding lots of compromise into the system when it need not be there. I would guess the 1:4 is needed for the occasion when one needs to "turn it up to 11" and if that is the case it should only be used when needed and not be in system all of the time.
Having a 390 ohms source drive the autoformers is fine and the output impedance from the autoformers will always be 390 ohms or lower so the source will have no issues driving the step up when needed however going the other way around and feeding the autoformers from a 6K+ source impedance is not placing them in the best situation. If you find you constantly need the extra 12dB of gain then a better solution would be a simple active stage.
dave
Having a 390 ohms source drive the autoformers is fine and the output impedance from the autoformers will always be 390 ohms or lower so the source will have no issues driving the step up when needed however going the other way around and feeding the autoformers from a 6K+ source impedance is not placing them in the best situation. If you find you constantly need the extra 12dB of gain then a better solution would be a simple active stage.
dave