Current Mirror Emitter Degeneration
From reading the Audio Power Amplifier Design Handbook by Douglas Self (ISBN: 0 7506 56360), it seems that unlike Vbe matching available to Integrated Circuit (IC) designers, discrete transistors need emitter degeneration for good current matching. "A voltage-drop across the current-mirror emitter-resistors in the range 30-60mV will be enough to make the effect of Vbe tolerances on distortion negligible". Some say "100mV... but this is not critical" (like Bob Cordell, Designing Audio Power Amplifiers, ISBN: 978-0-07-164025-1). This PDF says to maintain Vdegen of at least a few hundred mV. In this example there is 1mA flowing through each emitter, so a 100Ω resistor will drop about 100mV.