Class AB Amplifier Frequency Response
The basic answer to this is to start by making sure all the signal paths have only high pass filters with rolloff no higher than 20 Hz, and low pass filters with rolloff no lower than 20 kHz.  There will still be some uneveness in the frequency response, but it shouldn't be drastic.  Take a close look at what filters are formed by any caps, and make sure they meet the above criteria.

Then make the open loop gain about 10x or more of what you want the closed loop gain to be.  That leaves room for some negative feedback to do its job.  Don't go crazy with gain, as too much gain will make it difficult to keep the amp stable, and too much feedback to get to the desired gain introduces transient intermodulation distortion (TIM).

Then put feedback around the whole amp.  Again, this hopefully doesn't need to be more than 20 dB or so.