First take at a Micro amp - a 2 watt parallel-triode powered Tweed Princeton made from junk I had laying around. A fun build, I've have to try another someday.
The sound you hear when you put the feedback resistor is oscillation. You’ve made the feedback too strong when using a small resistor. Try leaving the resistor there and wire a 100k or more pot in series and check what’s the value at which it stops oscillating. Then place a resistor of that value there.
The "proximity" of your parts/layout is not your problem. I've built many tighter and higher gain builds w/out your issues. Motorboating is a fairly common experience and means you've created oscillations. Your layout is not too bad but the grounding is far from optimal for sure. Without a schematic it's hard to follow what you've done regarding possible mistakes on how you made your layout. It is bad form to run your 1st preamp tube's cathodes' grounds connecting to your output transformer's ground the way you've wired it. Run the output ground directly to the star ground. Those cathodes' ground and your input jacks' ground should be ran straight to your power cap's node grounding point aka the neg lead. Proper grounding is an art in itself but critical to high gain circuits. Yours is not a high gain circuit and one of the easiest simple circuits. It's hard to tell if you're missing any grid leak resistors but could be your issue as well especially for the 6sn7 "power tube"? I assume you're running it in a self-split mode like similar Firefly amps.
Have you tried grounding the input jack, like on the original the jack is grounded when the guitar is not plugged in. Just a thought.
The sound you hear when you put the feedback resistor is oscillation. You’ve made the feedback too strong when using a small resistor.
Try leaving the resistor there and wire a 100k or more pot in series and check what’s the value at which it stops oscillating. Then place a resistor of that value there.
sounds like input polarity
The "proximity" of your parts/layout is not your problem. I've built many tighter and higher gain builds w/out your issues. Motorboating is a fairly common experience and means you've created oscillations. Your layout is not too bad but the grounding is far from optimal for sure. Without a schematic it's hard to follow what you've done regarding possible mistakes on how you made your layout. It is bad form to run your 1st preamp tube's cathodes' grounds connecting to your output transformer's ground the way you've wired it. Run the output ground directly to the star ground. Those cathodes' ground and your input jacks' ground should be ran straight to your power cap's node grounding point aka the neg lead. Proper grounding is an art in itself but critical to high gain circuits. Yours is not a high gain circuit and one of the easiest simple circuits. It's hard to tell if you're missing any grid leak resistors but could be your issue as well especially for the 6sn7 "power tube"? I assume you're running it in a self-split mode like similar Firefly amps.