Negative Feedback Portion of Amplifier
I am wondering if the difference in amplitudes between two
  base-to-emitter voltages of TR2 and TR3 is increased if we increase
  the Rf1?


The open loop gain of this amplifier is finite so, to drive a bigger output signal for the same input signal, something has to increase and that will be the difference in signal amplitudes between TR2 and TR3 bases.

It won't be much because at 1 kHz (for instance), the open loop gain will be 1,000 to 10,000. With this sort of gain, the base signal difference to drive 10 Vp-p at the output is in the range 1 mV to 10 mV.

So, with a closed-loop gain of 23, to drive 10 Vp-p at the output, the voltage at TR2's base will be 435 mVp-p and the voltage at TR3's base will be 1 to 10 mV lower. If closed-loop gain doubled, there would still be this voltage difference but TR2's input voltage would halve to achieve 10 Vp-p at the output.