These capacicitors have been recently checked and are in their correct
values, also in case that is the reason for the problem; Wouldn’t it be
present on both channels?
Correct value isn't so much the issue as whether or not the caps are drawing power. Usually you would expect power supply issues to manifest in both channels, but when the bypass isn't correct that isn't a given. So if the caps are original get them replaced- if the power transformer dies you'll really be regretting it and they smell terrible when they go.