New caps for preamp?

Preamp works & sound fine but is 14 yrs old.
Would new caps improve sound?
If so why, how.
If the amp was always working for all these years than most likely you'll need to worry. Otherwise I would check/recap the amp especially electrolytic caps. They're most-likely dry and need to be checked with either cap tester or C-meter for the specified parameters. Each of these caps may cost around a dime(no need to shop for the 'improved ones' the originals are the ones the preamp is specked out for) at parts retailers and would extend the life of your unit by another decade or more.
That would require knowlege of testing and replacing electrolytic capacitors from the circuit board. Giving the unit to the local tech would hit your pocket much harder if you figure...
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.
OOPS- I re-read my previous post, and should have said the ARC would already have excellent Wonder Caps in the signal path. Anyway- Welcome to the world of upgrading(maybe).
Capacitor upgrades are one of the better ways to improve component performance.. The next upgrade would be resisters in the audio signal path.. it will improve speed, transparency and micro dynamics and is well worth it.. One of the main differences between the Mega Buck components is they use the best capacitors and resisters (clearly for a reason)... It is however very important to only replace them with the same exact values.. Some of the best caps I have encountered are Teflon Film Foil units, there are a few one the market but the Solen Green cased caps (yes Solen) have the best price performance ratio... They sound best after about 150 Hours of use.. On resister issue I think Tantalum are the best, Tantalum is not a brand however its the resister element used ( a rare metal ).. There are also some great sounding NOS carbon units as well... Research techniques and procedures a little If you are going to solder them in yourself... and have fun... Peace