For #1 it was based off my original modification from Speakers too Quiet? Try This - #4 by Chartreuse where I replaced the two 47uF MLCC capacitors with first 100uF then 330uF electrolytic capactiors to upgrade the bass response. It’s a bit fiddly soldering those through-hole caps to the tiny 0603 pads, and you then need to make sure they’re solid and supported as to not rip those pads off afterwards.
220uF is a good compromise on size as with say 24ohm headphones (16-32ohm is common for consumer ones) that’d give a bass rolloff of 30Hz (-3dB point), much better than the stock 141Hz
You’ll want to change both C127 and C147 otherwise you’d have an imbalance between the two stereo channels.
A simpler version is what @minute did as a quick test which is to piggy back another 0603 47uF ceramic capacitor ontop of the existing one (Rolloff of 70Hz), though it’s still a little fiddly to do, but not as large. And they later added another on top to bring it up to around 141uF ish. (Rolloff of 47Hz)