Thank for the reaction. The stages are C-coupled and they're not leaking. After the first stage and cap however the second half of the paraphase is driven (signal applied to the grid, attached to ground via 100 k resistor). Indeed somehow here is the result a -4 Volts which passes to the grids of the output tubes. It is not the grid that is drawing current however.
If I take the ecc82 (very first stage) out of the amplifier (and effectively make the amplifier open loop) all problems are gone. It has to be a oscillation (I have no scope). But if this is true, all AI200 must have this problem!