BC550 Darlington Clipping

The problem in your design isn't the output transistors and neither will this be solved by using darlingtons - in fact the problem will become worse with darlingtons. Look at the spec of the NE5534 and read the section about maximum output voltage swing.

On page 5 it tells you that the max swing might be as low as 24V on a +/-15 volts supply i.e. a 30V supply only guarantees an unclipped output swing of 24 volts p-p - that means the internal transistors in the device "lose" 3 V top and bottom. This doesn't magically improve on a lower supply and you should expect, on a 9V supply, to generate a maximum output swing of 3 volts peak to peak.

OK, if you used the typical spec of the NE5534 this might rise to 5 volts p-p.

This is then compounded by the push pull stage - it's an emitter follower push pull stage and it's output will "lose" between 0.6volts and 1 volts per transistor depending on your load.

This leaves you with a crappy 1volt to 3.8 volts p-p being fed to your headphones before clipping takes place. If you use darlingtons, they "lose" double what a regular transistor loses so this will be even worse.

Recommendation - use a rail-to-rail output op-amp. There are plenty around these days and they'll probably only drop about 100mV per rail.

Your first two transistors look incorrectly biassed so that Ve is about (0.5*Vcc - 0.7V) in Q0, Q1 maybe slightly better.

This means (thanks to symmetry between Re and Rc (e.g.R0 and R3) that Vc is about Vcc/2+0.7V, in other words Vce=1.4V approx, and the transistor is close to saturation.

First step : run a DC analysis to confirm this for both common-emitter stages (the emitter follower is fine).

To maximise the linear dynamic range you want to maximise the possible collector swing, so as a first pass, re-bias for Vc=9V, Ve=3V, i.e. Vb = 3.7V, calculate the base resistors for that - and see if it's improved.

This gives you up to 6V p-p swing at the collectors. It's not the best you can do as you have less AC swing at the emitter therefore you can increase headroom further, but I'll stop here.