The relationship is logarithmic. For every increase of 3dB power doubles. For 10dB power increases ten times.
In your case switching between 23 and 29dB of gain is 6dB. So whatever input it takes to drive the amp to full power at 23dB, it will reach full power with 1/4 that level when set at 29dB.
In your case switching between 23 and 29dB of gain is 6dB. So whatever input it takes to drive the amp to full power at 23dB, it will reach full power with 1/4 that level when set at 29dB.