I boxed this one up today, it's going to Wales.
I can crack on and finish Jochim's.
Sandro, what freq is it oscillating at? Have you tried putting the decouplers on the heatsink? 0.47uF film cap direct onto the underside of the board, from the emitter to ground on the PNP and the NPN on the outside of the heatsink. Essentially they do the same job as the electrolytics on the psu input (which knocks put cabel inductance effects) but they're local, and sized to mop up HF in mHz range.
Show me what you've got.
Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m