A standard method to find out if a tube is at fault is to switch the tubes in the left channel to the right and vice versa. If the noise switches sides then you know it is probably time for some new tubes.
Two caveats. First, sometimes the noise is present because the tube is seated poorly or some pins have some corrosion. Removing the tube and reseating should cure this problem. The second caveat is only work on one device and one pair of tubes at a time. If you change too much at once you won't know which exact component is involved.
If the noise does not switch sides or disappear, then you know it is not the tubes.
Keep in mind that just because the amp was serviced recently does not preclude the caps or any other part. No matter how tight their quality control, every manufacturer will have a few parts that seem to pass but contain a defect just underneath the surface and fail sooner than they should.