Been watching yr progress and looks OK.
There seems to be a failure on the part of the design team to establish the;
1. Input voltage to drive the amplifier to full power.
2. the preamp maximum output.
I aint yet seen a clean sine wave output yet, and I haven't seen a biasing system to set the working bias for the output stages.
I note your 4 diode arrangement, which is essentially a non adjustable bias, but this could be done by using a single transistor. If you replaced the 4 diodes with a few resistors in series, an NPN emitter connects to the lower side of the existing diode D4, and the collector connects to the anode point of D1. Using a potentiometer in the chain of resistance, you connect the transistor base to the moving arm of the pot. There needs to be an AC short circuit across the bias network, so shunt the emitter and collector with a capacitor.
To set the bias, you need enough of it to get the output transistors JUST into conduction. A typical value is around 20 to 50 mA. This is described as Class AB. Without some forward bias, there is the danger that the amp will produce crossover distortion and this is very objectionable to a listener.
There seems to be lots of comment about the ability to overdrive the power amp. When the team agrees on the input voltage requirement for full output, then go to work on the feedback network and keep on increasing the feed back until the amp gives full output at the required (agreed) input. Typical commercial amps use between 100 and 400 mV for full output.
Looking at your circuit, my view is that there is nowhere near enough negative feedback. These things usually have heaps of it to stabilise the DC conditions against drift due to temperature, and changes in the working point due to 'rectification' effects. Looking again at your circuit, you have a tapped power supply. Connecting a loudspeaker to such an arrangement is asking for trouble if, for some reason, the speaker gets connected to the DC supply (eg, an output transistor goes short circuit). I suggest you use a output capacitor in series with the speaker to prevent a disaster. This capacitor can function as a low pass filter if you use a small value.
Of course, testing the amp needs a means of adjusting the input signal level and I note you are incorporating a volume control. When the design is complete, just leave it there set to maximum.
Hope this helps and it looks like yr all going well. So congratulations so far.