Power amplifier input source current path
I will try to show you two extreme cases (when clipping). 

For the Positive half of a sinusoidal swing:



And for Negative half of a sinusoidal swing:



This two cases should help you understand what is going on in the amplifier. 

Because in the normal operation we have "slightly" similar situation. 

For example:

For the Positive half, \$Q_1\$ reduce his \$I_{C1}\$ current (just a bit), hence \$Q_2\$ must increase his \$I_{C2}\$ current. 

But this reduction in \$Q_1\$ current decreases the \$Q_5\$ base current also. 

And all this means that the \$I_{C5}\$ collector current is decreasing also.  

Hence, the voltage at \$Q_{12}\$ collector increases. 
The larger part of a \$Q_{12}\$ collector current (\$I_{C12}\$)can now flow into the \$Q_7\$ base. So, \$Q_7\$ and \$Q_9\$ conduct more current. And the load current increases.   

I hope this helps you. Because my English vocabulary is limited.