Well having messed around with input caps I've decided to proceed as follows.
1. Turn the guitar down so it doesnt distort. Its loud enough anyway for living room jamming.
3. Use the preamp and EQ pedals which I already have.

Actually an eq pedal or other pedals with tone controls work well but I'd like to make it standalone. I know there is an impedance mismatch but I find the treble end  acceptable. Going to try just the capacitor on the input as a starting point.

Guitar straight in.

Im using one of these built into a bookshelf speaker as a practice amp. It works great with electric mandolin and harmonica but the speaker gets overwhelmed with guitar turned up loud. I was wondering if there was
a way to filter out some of the bass?

https://www.ebay.ie/itm/374178708497?mkcid=16&mkevt=1&mkrid=5282-175127-2357-0&ssspo=YI5yZF5vTzy&sssrc=4429486&ssuid=5dZ_imPdR-i&var=&widget_ver=artemis&media=COPY

I had this model and got a tech to successfully repair it by changing the Opamp. If you have the skills it might be worth replacing it, using a socket so that it would be easier to repair again.

But then some pots and PCB traces failed and I made it worse while trying to fix it. I decided to gut it and made it into a hybrid head, plus a handy, bright orange,  1x6" cab for other projects. The power supply section is still useable, I think.

A pity in a way because it is probably one of the better small amps in that price range.

There's one of the above for sale locally for €25. Selller doesn't even know if it works. Worst case it's a useful enclosure, pots, jacks, maybe power supply. Any thoughts?

Great to read stories of people modifying gear like this, and also handing it on to a younger generation. My kids showing no interest in guitar gear yet, sadly. I'm holding out for their  kids.

Where would the low ass filter go?

What software do you recommend for drawing schematic,  free or not?

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By the way it's all running at 12v dc and I have left out the tone pot.

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Noise was worse when I wiggled the gain pot so I replaced that which seemed to solve the problem. The whole thing did work fine at first and the preamp and power amp each worked fine on their own so interested to know if the noise was due to the power amp getting too much signal or not.

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https://1drv.ms/u/s!Ahfhrrb-UXCanS2OqSaG-XLKGiCx

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Heres the noise which comes in as I increase the volume.  https://drive.google.com/file/d/1KL_uZ8kQ9_aCp2k0lP7rCePaSFM_Zxf0/view?usp=drivesdk

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Did you make any progress on this? If you are like me, such projects happen over months rather than days. I have had good results with pedals straight into a power amp module. The Kemo amp modules are durable and stand up to soldering better than very cheap ones. 

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One reason I use LM386 is that they provide an acceptable kind of distortion on their own. Will you be able to achieve that, or will it be a clean amp?