It was Q402, shown on the Sense PCB Sht.4 schematic.
Spot on! I decided that your hint would be the first place to start troubleshooting and yes, it was failed Q402 too.
At a closer look I've noticed somebody already replaced this MOSFET (flux residue) and with 2N7000 instead of BS170 that is specified in the service manual (remember to flip the transistor around - different pinouts). Fortunately, I had some spare 2N7000 and the amplifier works now.
Now need to calibrate that. For DC I can use the L&N 4385 shunts. For AC I don't have a thermal transfer standard, so Keithley 2001 will be the next best thing I guess.