AB class amp, design considerations
That circuit is rather simplistic and a bad idea.  It's clear enough how it's intended to work, but the implementation is lacking.  In particular, the lack of emitter resistors in the output stage is downright irresponsible.

I was going to go into detail on how this circuit works and how to tweak it, but now realize you didn't provide component designators.  I'll be general and brief as a result.

The input is a differential pair.  The signal is fed into the positive input and feedback from the output into the negative.  The two feedback resistors set the overall gain.  The output stage is two complementary emitter followers.  This would normally have a input dead band equal to the two B-E drops, but the diodes are used as voltage sources to (largely) compensate for that.

The irresponsible part is that the output stage is biased open-loop.  Consider what happens when the output transistors get warm.  This is pretty much guaranteed to happen because those are the parts that will dissipate most of the power.  Their B-E drop will go down.  However, the diodes aren't dissipating much power, so their voltages stay the same.  This will cause more current thru the output transistors, which heats them more, which causes more current, etc.  Again, this is a crappy circuit, but wouldn't be hard to fix.