dc psrr
Your forward gain in the amplifier, is only one part of
the story (and perhaps not the significant part). Look
at your bias racks and other supply-to-output paths
which "should" be supply voltage independent, but can
never -entirely- be.
Preregulating with a crude LDO is one way to bump up
PSRR, if you have the headroom. This regulator need
not be DC-fabulous, just well AC-decoupled.
You can also post-filter the bandgap with an RC if
you can tolerate a weakening of the drive (load reg
degrades, but something that only has to drive a
capacitive load might not care).