Bass amp speaker

[loopyloo]

The trim pot at the front is for DC Offset. With the Input to the power amp shorted to ground, adjust for 0v at the speaker output.
That said, neither should need adjusting.
Look at the preamp section for your problem.
To check, short the input to the power amp board and then switch on and off.

I just wondered if the trimmers could be set wrong (contact degradation) due to being in the one spot for a long time. I could spray them, turn the wipers and put back where they were if I knew they would then be right again.

To short the input, do you mean short the guitar input jack ?

PS just re-read your post and I see what you mean, I'll have a look inside it again.

[Uncle Fester]

Don't touch the trimmers until you know what you are doing. They also do not seem dirty or oxidised. Just moving them back and forth might mess up a few things.
First measure if there is any DC on the output. Less than 20mV I wouldn't touch the trimmer, even 50mV for now.
That circuit is so spartanic it might be even rather difficult to get it right and temperature will change things too.
The bias (idle) timmer next to the heatsink would only need adjusting if it sounds distorted when you play very quietly or if the power transistors get rather warm after it has been switched on for about 10 minutes but you haven't played yet. A slight bit warm is evidence that the bias current is good. You can measure over the big white resistors. Because I can not see their value I can't tell you what you should measure. It will be in the low millivolt range.
This certainly has nothing to do with the thump.

What does have to do with the thump is the primitive design and most likely that small black electro cap.
It doesn't matter if you short the imput. The cap could be for a simple bootstrap circuit or just DC decoupling from the imput and in either case it takes some time for things to stabilise while it charges up and output settles to zero.
That is your thump.
There is really nothing you can do there it is designed like that, except adding a relay delay circut to the speaker.
This may or may not have something to do with the damage on the suspension but I assume cockroaches(the small German type could fit through) could have had a nibble too.

[loopyloo]

OK I'll go with the relay then.

Found one on Banggood for $9. At least I know it will get here in 2 weeks, well usually.