more impression on KSC55 drivers with 1g acousta-stuf stuffing:
* good bass, goes deep and has solid impact. Actually a little too bassy at times (and this is coming from a self-proclaimed bass-head)
* voice can "bite" at times, slightly sharper than I'd like.... but I suppose I shouldn't expect too much from $15 drivers.
* very engaging. Can keep on listening for a couple of hours at a time ( with stock phone I can do 3~4 songs, maybe)
* The detail is good but not great. However I don't feel the details lacking unless I know something "should be there".
Overall, sounds pretty ok for everyday work use.
Lets call this mod level, "driver-transplant-only and no output cap upgrade", the Level-1.....
* driver transplant only, leave the output caps alone. Very easy mod, without the risk of overheating the circuit board and lifting the pads (guess how I know).
* new drivers should have high sensitivity and >30 ohm impedance (for deeper bass than stock).
* recommended drivers: start with with the Koss KTXpro/KSC55 drives and see how you like the sound, if you want more warmth and less brightness, go for Sportapro drivers; for more brightness and less bass, go for KSC75 drivers. These Koss drivers are 60 ohms.
* add some acoustic stuffing to kill echo,
Tools and parts:
*screw drivers and soldering iron.
start with drivers from Koss KTXpro1 ($14) https://www.amazon.com/Koss-KTXPRO1-Titanium-Portable-Headphones/dp/B00007056H
* acoustic stuffing: a few grams of acousta-stuf, or some foam, or some cotton balls, or , as has been suggested, some cotton pads. 1 gram of acousta-stuf per ear cup is working ok for me.