I have one of these, my sleeper, as I call it. If memory serves me, I couldn't adjust the bias to a low enough level and ended up replacing the diferential drivers, Qe1 & Qe3 and Qe5 & Qe7.
KSA992 Transistors, PNP and KSC2690 Transistors, NPN. Also replaced the trimmers to Bornes multi-turn...