How to make this circuit unity gain?

[WHONOES]

The reason that adding the capacitors have a large effect is because they performing what is commonly known as decoupling. Wire has resistance and, importantly inductance. As we should all know (simplifying it a bit) inductance resists the flow of current when the demand is a simple pulse or a continuous frequency. The several amps that your psu may be able to supply will not be available instantly due to the inductance and resistance of the wire. Even a shortish length will have some effect. By placing a large capacitor electrically and consequently physically close to the output transistors, makes more instantaneous current available.
As for a discreet design for the headphone amp. Should be possible. I will get my thinking cap on.

 

[AFex54]

really helpful everyone, learning these smalls bits of electronic theory really adds up, knowing just a few pieces of the puzzle makes the whole thing a lot less obscure.

 

[AFex54]

I forgot to ask should the 22r emitter resistors be rated at high wattage like the original 5w 100r emitter?

 

[WHONOES]

Just did a very quick sum. Dissipation in each emitter resistor is about 220mW for 100mW output. 0.5W or above resistor should suffice. You will soon know if they are under rated cos they will get hot.

 

[WHONOES]

Just came up with a discreet headphone amplifier. For reasons of stability (which I won't go into) It has a gain of about 4. If that is too much then pot down the input voltage.
See attachment. It has a lot of notes on it that you should read.
It will output a little over 5W rms into 8Ω so may be of interest to others.
The output will need to be on a small heatsink if run at that level but I would advise doing so anyway.
Let me know how you get on if you give it a try.
Other versions are available.