A lot depends on the make of your pre-amp. If it happened to be an ARC for instance; the unit would already have excellent MIT caps in the signal path. But- If you were to replace the power supply's electrolytic caps with Black Gates(there are still some out there), or Sanyo OS-CONs; you'd still realize quite an improvement in sound. While you were in there, you could replace the bridge rectifier with a soft recovery FRED bridge. There were quite a number of manufacturers turning out high quality caps(polypropylene/polystyrene and foil, etc) 14 years ago, and the builders of high-end pre-amps were using them in the signal path(at least) and/or as bypasses too. Cheap caps in a power supply= crappy sound, Period! No need to stress about exact replacements. As long as the values and voltages are matched(or exceeded, in the case of voltage); you're good. It would help to know what caps you've found in your circuitry. Better recommendations could then be made.
