The consensus of every knowledgeable amp tech I know - including those who've checked in here - is that you replace electrolytics that are over 10 or 15 years old. You replace the less expensive short-lifetime parts to protect the expensive parts. Perhaps you haven't seen shorted e-caps take out a vintage transformer, but there are others who have and this is pretty common wisdom. There are exceptions to this rule - the typical failure mode of e-caps is worse when equipment has not been turned on for a long time - the reason e-caps are polarized has to do with the chemical reaction in the electrolyte which is maintained better with regular use, and when caps are said to be "dried out" it's because the electrolyte no longer is capable of depositing an insulation layer on the electrodes. So if an amp is in regular use, it is somewhat less likely to fail this way. Anyway I'm more of a theory guy than a working repairman, but the advice from working repairmen is pretty consistently to be safer than sorry on this point. The analogy I see a lot: "would you drive a '66 Mustang on its original tires?" This is where my comments are coming from.