I have a Stern Meteor that sounds like it's got a rolled up sock in front of the speaker. Sounds are dull and muted. Very little treble/high end, as if there was a low pass filter on it. It gets loud- overall amplitude isn't the problem. It just sounds really dull and muddy, without brightness.

Sounds themselves are fine. Everything plays when and how it is supposed to, but the quality is very poor.

Has been re-capped and the problem isn't the speaker itself. I could start tearing apart the audio amplifier circuit soon but I figured I would ping the crowd here first. Hell, maybe its supposed to sound like that.

That's a good assumption- the electrolytic swap had no effect other than perhaps a bit of futureproofing and cosmetic improvement.

TP3 is steady at 9.75VDC. I'm not unhappy with that.

I didn't even recognize these glass-packaged capacitors as capacitors until really squinting down at the schematics. I'll focus on them next.

Looks like you may have been on to something.

Looking through the glass of C12, it's dark and toasty compared to C3. If they were both installed at the same time, something went wrong along the way. Unfortunately, the capacitance meter is in my workshop on the other end of town, but I'll order the parts anyway. I go through a lot of these caps anyway.

Popped in an order at digikey for all of those axial ceramics. Can never have enough 0.1uF. Waiting on an order of radial ones from overseas, but why bend leads when I can make it look nice? Added a few resistors too... R3(3K9) and R4(10K) are way off label at 3K and 5K respectively.

And for the real hell of it I ordered a few MC3340P and TDA2003A chips from AliExpress. Possibly counterfeit, hopefully just old stock, but they're so cheap that I feel like I'll roll the dice.

Well, I pulled all of the passive components from R23 onwards, and it's still muddy as all hell. Perhaps my old ears deceive me.

Good timing. I was just putting in a digikey order for some electrolytics. I'll put in a BZX85B10 as it has a much tighter tolerance than the original 1N758. We'll see how it goes!

A_Bord: when I get this licked, you can be sure I'll post the solution here.

Replacing the Zener diode VR1 made a huge difference. Solid on 10.01V instead of a shaky 9.6-9.75V. Made a big difference in sound levels.

I still don't feel like I'm getting the highest highs out of it, but then again, it's an SB-300. Not exactly high fidelity. Still, a major improvement.

In any case, massive help. Thank you.

While I was under the hood I took out the scratchy 25K pot that was always cranked to the max with the 20K ballast resistor and replaced them both with a 50K pot. Solved any possible concerns I could ever have about volume. I get the full range now from nearly silent to unusably loud.