a resistor in input circuit is confusing me

[john-china]

i saw , in input circuit A30 uses 68.1k(R5,R6) in stead of 10k(R14,R18) of Aleph4. i know that using 68.1k will increase input
resistance but also increase feadback , so the gain of A30 will get lower.in my calculation the gain of A30 will be 1 because feadback reach almost 100%.(100k/68.1K). i want to know how to calculate
gain of A30 in balance input mode. i saw the gain of A30 in manual is 20db balanced , how to get this value? thanks

 

[Nelson Pass]

For single ended operation we short the (-) input.

 

[john-china]

thanks to nelson

i know the gain will increase to 10 times when short the (-) input.
because the 10k resistor parallel 68.1k. but i want to know why
gain will be 20db when using balanced input , and in the feedback circuit 68.1k is to the ground so the feedback will be too high because feedback resistor is only 100k. pls tell me how to calculate to this value (20db) when balanced.