My bad! Just a loose connection on the positive rail of the left channel. Weird.
Offsets now perfect ~4mV both channels.
Yes, you can bias as hard as you like. Just change the bias trimmer value for something larger.
It's pointless though - The improvement in performance is naff-all, and all you'll do is heat up the room.
This amplifier, with it's fast high open loop gain, is designed with reasonably low bias, to be one of the more efficient LatFET amplifiers.
A shorted Mica cap? As we used to say "Murphy was an optimist: He thought only things that could fail would fail." We came to understand that things that couldn't possibly fail will also fail! Or as a mentor of mine said "No, you won't see it in the units you build in the lab, you won't see it in a pilot run, but you will probably see it in the first 100K units, and you will certainly see it when you build a million of them!"