Re-capping A 15 Year Old Solid State Amp

AFAIK only electrolytic caps and possibly switches if you have problems with them. Everything else should last your lifetime. Old dry caps have high ESR (Equivalent Series Resistance) increasing output impedance of the amp. It will take away some dynamics and will make a woolly bass - at least that's what I observed when I changed power supply caps in my older SS amp. Whole thing started sounding much more dynamic with better bass definition.

I won't buy a 15 year old amp as its history!!

1. 15 years isn't old if it had been always used and i'd probably give it another 5...10 years safe.
2. Amp in need of recap may increase noise level when in idle or even start humming for no reason.
3. You may do visual inspection on how clean the caps look and how clean the PC board where they're connected and if you spot traces of heat or liquid, it's another sign.

Ebm,
Let me know when you have your old amp on curbside.

Ebm - In your statement "I won't buy a 15 year old amp as its history!!" all I can say is you must be buying throw-away junk!!. I have owned many Solid State and Tube amps much older than that from reputable companies (ARC, Conrad-Johnson, McCormack, Bryston, etc.) who use high grade components and there is no problem.