If you want to maximise the forum members' input, I think we're going to need more information, i.e:
Why are you uprating the original supply? Does it have a ripple problem? Are there RFI / EMI issues with the standard supply? Are the audio electronics renowned for poor PSRR (Power Supply Ripple Rejection)?
What kind of supply are you looking to implement? Solid-state regulation? L / C regulation? Discrete regulators? Cascoded regulators?
Do you feel the mixer has trouble driving other devices such as crossovers / amps etc? (This may not be a psu issue, it could be due to op-amp choice).
If the standard mixer uses an E/I transformer, you may well gain a noise-floor advantage to making it outboard.
Does the standard transformer overheat? Does it suffer from mechanical hum?
What voltage are the DC rails? I'm guessing +/- 15v?
Sorry to bombard you with all these questions, but it's best to ascertain the goals to optimise the solution.