Ralph- yeah, more like 3db, noticeable. But if there's no distortion, and not a blown output(maybe yes?), what could cause that? My main concern is the transformer.The fact that the power transformer is running warmer is a bad sign and I agree with others that the filter capacitors in the power supplies are suspect (and cause this sort of thing) especially given the age. If left unchecked it could lead to transformer failure.
It is also a good idea to check the bias points. This is not particularly hard to do; with the cover off a tech can do it easily in five minutes.
So there are the two things that really should be sorted and it could well mean that they sort out the problem too. But even if they don't this sort of service would be prudent.