How to improve power supply?

Ptss, how do you know it is inadequate? Most likely it is linear power supply followed by voltage regulators. AFAIK nothing changed in design or available regulators since mid 90s. There are few areas that might bring improvement. It might be possible to replace power supply caps with better ones, replace rectifier with one that has soft recovery diodes, add high frequency filtration of input voltage - either external (power conditioner) or internal (metal box modules or power socket with filtration). It is also possible that your pre has fine power supply but something else deficient. Why not to sell it the way it is?

The power transformer. Look into Plitron transformers. This update could be HUGE in clarity and dynamics.

If your power supply does have regulators, replace them with Bellesons, which are much quieter(http://belleson.com/download/Superpower_datasheet.pdf). Replace the rectifiers, or bridge, with with either HEXFREDs or Schottky diodes. Parts Connexion offers small PC boards, for building your own bridge with discrete diodes(if needed). Your filter capacitors could probably use a freshening up. Why not increase the capacitance, and pick up some dynamics? Keep that increase to 20% or less.