The one way to confirm if it's the filters is to read the high voltage while keyed, and see if it falls off in step with the output power falling off. This is trickier than it sounds. The RF from the tubes will cause many DC voltmeters to go psycho until you unkey. A 'scope is good, but most probes aren't rated for 900 Volts DC. You have to use the probe's "Times 10" setting to see anything this way.
Old filters will usually look bad if they're getting ready to fail. If yours are original, you should replace them just to prevent a "KABOOM" later on. If they show cracks, swelling, or a "zit" on the rubber end insulator, it's past time to replace. In a Varmint, the bleeder resistors should get changed at the same time. If one of them goes bad, it will clobber a filter. They mainly serve to split the High Voltage evenly three ways across the filters. If one of the bleeders goes bad, one filter gets more voltage than what it's rated placed across it. That filter will short out inside. Tends to clobber one or both of the other two when that happens. Cheap insurance, those bleeder resistors.
My money is on one or more tubes being the culprit. If you have access to a tube tester, you will probably observe the tester's meter reading fall off while the knob for the "emission" or "quality" test is held to that position.
There are other possible causes, but most of them will cause the "peak" position of either "Plate" knob to shift. If 'rocking' either plate knob does NOT return the lost power, you're probably looking at tubes. If this brings back ALL the power you lost, tubes aren't the culprit.
It's only necessary to overdrive tubes like that once or twice to make them behave this way. Once damaged, there's no way to "fix" this failure.
73
