How do you when you need new caps? The age of the cap should be the first way of determining if replacemts are needed. As far as hearing the difference, its subjective but what I can notice about old caps is a lack of focus in the soundstage. Its there, but in a somewhat blurry type of sound. Old caps can sound decent to some people, but when you install new decent caps, you will immediately notice an improvement in realism. A much sharper true sound, and more full sound. Some say new caps can provide a brighter sound, but the new brightness is just the sounds that were lacking from the original old caps.. so you are actually missing out on a full sound with the old caps.