[morzh]

I never check mica caps. The overmolded type rarely goes bad. Unless a cicuit malfunctions and you specifically suspect a mica cap, they don't need to be checked.

Going back to the volume. When you say "half as expected", how do you know what's expected?
With antenna or with the finger on the Antenna input, does a local station come loud? And, no, if the finger on it increases volume, it is good, means your signal gets through.

Here's another thought:
The amplifier in the tuner chassis has two parallel channels: Bass and Treble.
Bass is the 6K7G and then it mixes with treble in the 2nd Audio 6N7G.

Pull out the 6K7, listen to it with treble (the regulator CCW), then put the tube in, see if the bass comes in.