autoformer attenuator and step up transformers

-5dB

If I were to use a step up and autoformer combo I would place the 1:4 after the autoformer and arrange it so it is only used when it is absolutely needed.  

dave

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 

the perfect position for the autoformers is full volume then they are simply a shunt element.  The worst performing taps are in the 0 to -6dB range but they still have 100Khz+ bandwidth and what ringing there is is also above 100khz which is far enough outside the audio band that it doesn't bother me from a technical perspective.  In any event the behavior of the autoformers at -3dB will far exceed that of any step up that you put in front of it and if just a slight amount of gain is needed (up to +6db) again the autoformers can be configured to seamlessly shift into step-up mode so you not only get 11 but possibly 12 on the dial.  The important part here is that when you leave the "gain" mode you also leave the compromises incurred from that mode.

dave