@boxsoft
That preamp should put nearly a constant load on that rectifier. So Voltage sag really isn't a concern unless the rectifier is shot.
If the power supply is properly designed (bypassed), the character of the rectifier will be very heard to discern!
If this is an older preamp it might simply be that some filter capacitors in the power supply have degraded.
The actual audio tubes in the signal path are far more important to how the preamp sounds!! What brands are in there right now?