Please supply the name of the pedal and the schematic.

It appears as if the B- rail is missing.

So, either the power supply is not supplying or the switch is bad.

Of coarse you could have an open connection from the switch to the ic.

Just to double check that you have the pins correctly identified:
Amp Off. Meter on Ohms scale.
Pin 1 & 4 should be 100 ohms apart.

If Pin 4 is at the same potential as any of the tabs, that would possibly make it pin 10 instead of 4.

If I am not mistaken, the heatspreader has tabs that solder to the ground tabs.

"it's hard to see the orientation of the op amp in relation to the pins"

Every IC ever made has a marking at one end. (in this case, a divot)
That end has pin #1 on the left.

You could use a volt meter to find out what is wrong.

First, check the output ic to see if it has the correct voltages.
TBA810 power pin: 1 is +Vdc & the tab should be grounded.
Pin #4 is the bootstrap pin. It should have +Vdc on it.

Set your meter to read Volts ac & look at pin #8: that is the input pin.
You should see some action there when the input is fed a signal.

Pin #12 is the output pin.
Same thing: Volts ac at the output with a signal.

The output is capacitor coupled to the speaker.
So if you have signal at Pin #12 but no signal at the speaker, then that cap (or the speaker ground connection) is suspect.

Thanks for the info, Stevo.

Sorry if I got the amp model incorrect.

The Boost Output is another Normally Closed contact.

Use a stereo jack & jumper tip to sleeve.
Or send the tip & ring to another amp.

The Recording/ Headphone is an open contact jack so there are not any issues with the contacts preventing signal from passing through.
That jack could be sent to another amp or headphones to see if it is a preamp problem or not.

I would expect about + - 18Vdc coming out of the bridge rectifier.

Before replacing Anything, check that the speaker lead has a good connection to chassis ground.

The Ext Spkr Jack has a Normally Closed  contact that provides the speaker it's ground connection.

Insert a cord in the Ext Jack & jumper the tip to ring.
If the sound is then good, the jack needs work.

The resistive element should read around 1 Meg ohm when dark.

The reading should drop (not too sure how far) when light shines on it.

To find out if it is a swiching problem, unplug the amp from the outlet with the amp on.

If it then does not pop, you could try adding a capacitor across the switch legs.

"I am thinking about using a choke for the power supply of a TDA7293 amp as a means to filter out all that DC ripple"

What ripple?

If you have no idea what the replacement may be and the parts work, I would suggest that you leave them be.

If there is nothing wrong with the diodes, then you will gain nothing by replacing them.

"Now, when replacing these non-polarized caps I'm a little thrown off with "stripe indicates foil side down." With modern non-polarized caps we don't have to think this way? Correct? Either "

Yes, you are correct.

There no longer is a marked 'foil side'.