an amp more than 25-30 years old would have, at some point in time, been on a service technician's desk for some reason or the other. the key here is to ensure that replacement components were of good quality and of matching value. it is common for indian service technicians to use locally procured spares and even compromise on value. for example, if a electrolytic cap5 uf has to be replaced, if not available, the technician is likely to use 4.7uf. these little variances have an impact on performance. keep your eyes and ears open