pull both tubes and check voltages. Compare them to a sams or schematic. Just use small probes, you wouldn't want to short the voltage running through those components
If the coupling caps in this set have not been replaced, do so now. It will save you headaches trying to troubleshoot problems like this. i like to bring a set up to snuff before trying to troubleshoot. then at least I have many less problems like dried up caps, to worry about.
unfortunately you need to pull the whole unit from the cabinet to do these tests. Yeah, it sucks, but its a big help when trying to troubleshoot the wire side of a chassis.