[GrimJosef]

Polystyrene capacitors are actually among some of the most stable. If you can it might be worth extracting a few and checking their values with a dependable meter. You might find that they're no different from whatever you'd replace them with. In my experience the major problem with polstyrenes can be their physical fragility. When it comes to desoldering them it's all too easy to melt the plastic and/or inadvertently pull a lead out of the cap body. It's worth being very gentle with them.

As far as frequency response goes, 'flat' is normally what people aim for, and that's usually easiest to achieve in the middle of the spectrum. If your mid-range sounds flat then I guess that's the same as saying that the extremes (bass, treble) are either boosted or cut. That might well be the case if the tone control components have drifted. Can you find settings of the controls where the whole spectrum sounds uniform ?

Cheers,

GJ