Interesting, this schematic  increases gain
+ bais of the 310 stage, when the feedback
is set to zero. 

However what I see missing here,  is a frequency
compensation (cut off) for the feedback,  
so oscillations COULD arise. Specially with
excellent output transformers, you could have
this unexpected problem.  This is a bit of
an error, but easy to fix. 

Note,  KR300B is not a good 300B because heater
current is much too high for a 300B.

So before you put in a real 300B, or EML300B-XLS,
make sure you check the heater voltage. If 
above 5V, you should reduce it with a small series
resistor in the transformer 7V AC winding. 

Best regards,