Gain is not set by biasing. Biasing will change the type of amplifier from class C (no bias) to class B (just biased on) to class A (fully biased so the transistor is on for each full cycle). Class AB is push/pull or, of you like, two opposing class B amplifiers usually with a PNP and an NPN in the output stage but doesn't have to be. This amplifier is class A by the way. Class A amplifiers are the least efficient and so transistors usually do get hot. In this amplifier, the voltage gain is set by the impedance difference between the emitter circuit and the collector circuit. Because the caps across R4 and R10 have lower impedance at higher frequencies, the impedance ratio is higher at higher frequencies therefore the gain. You don't use opamps to power speakers. Tha's just crazy but, in this case, you would do better to just ditch the transistors and the rest of the circuit altogether and just use the opamp.