AB763 PI mods?

[TUBEDUDE]

A small negative supply applied to the P.I. tail instead of ground is an option if modding is on the table.

[cdemike]

A small negative supply applied to the P.I. tail instead of ground is an option if modding is on the table.

I've seen a similar approach in the Mesa .22 an .50 Cal series, though there it seems like the general operation is pretty similar to a plain LTPI sparing the effect of (im)balance viz. the tail resistor. Would the idea be to increase PI headroom by increasing plate-to-cathode voltage? Kind of like artificially raising the effective B+ at the PI node?

[Helmholtz]

It doesn't make much sense to mod the PI without evidence that the PI is actually limiting clean output, i.e clips before the power stage. Which I doubt if the amp works as designed.
My first question would be, what clean output does the amp measure?