The difficulty setting the V(R42) to 6.55V could be due to something else. I assume that you observed different pin out when substituting 2SA1486 with 2SA1413. As long as the LED strings are lit stably and not flickering, the RV2 should adjust smoothly for 6.55V across R42. The idea is to set the 2SK246 ccs to about 297uA with the majority of that going through R42 and only a tiny bit from the base of Q16. If you have some spare 2SK246 from the same batch, resistors and pot, breadboard a circuit and test with a bench DC power supply. See if the pot hits the bottom or something wrong with your 2SK246.
It’s a good idea to measure around the active battery circuit, using the top node as the reference point, mark voltages and currents on the schematic. The active battery does not take rocket science to troubleshoot. You just need to know 1) Ohm’s law, 2) Transistors have hfe, and 3) A forward-biased silicon junction has a voltage drop about 0.6V.
There is no need to be paranoid about the battery not being spot-on at 740V. Since it is transparent to the audio signal, they can even be purposely set slightly differently from each other to compensate unbalances elsewhere in the circuit. But if your amp don’t like them being near 740V at all, that means some other parts of the amp is quite different from what the designer has envisioned.
There are quite a few feedback loops in the works that maintain a near-zero offset. Let go of the active battery voltage temporarily in exchange for zero offset can set those loops at their normal operating point and help you find out who’s not behaving correctly.
Mark any voltage reading with a ‘*’ if you hear oscillation when you get the reading, as it may not be reliable.