A distortion analyser plus a scope can be very useful. You are going to need the distortion analyser for a clean audio source anyway. If you put the scope on the residual output of the analyser, you can "see" the source of the distortion. That is, with a dual trace, look at where the distortion residual signal gets large. If it is near the zero crossings, then the problem is in the AB stage. If the distortion is near the peaks, then there are issues related to signal swing or the stiffness of the power supply.
If you can track down an Audio Precision, that would be the way to go. Next up in the old HP8903B.
I guess I should mention that once you use the distortion analyser to knock out the fundamental, then whatever tool you put on the residual output is now much more useful since you are only looking at the error of the amp, not the full output of the amp.