I still haven't found the rest of capacitors available to buy in normal prices, so I decided to use what I have, at least to change one pair - 0.22uF. I've used Vishay/BC 0.15uF + Wima 0.068uF in parallel, giving me 230nF - close enough. They did fit but it is really tight fit.
Overall performance has improved after those few swaps so far but to make sure I'm going to compare fully modded amp with unmodded if I can find around here.
I have measured voltage with fully charged battery and compared it with voltage while connected to USB port. The difference was around 5mV. Also have compared sound with fully charged battery and USB connected to amp on its own - I can't hear the difference. I would say the better bass response is to do with battery discharging (mind that Li-Ion battery swings from fully charged to discharged from 4.2V to little bit less than 3.7V) which is 0.5V and this might cause difference in sound.
As for higher voltage mods etc, The 5V voltage step-up pcb would not fit inside, there is no space. I would suggest if one is brave enough to:
1. Take out battery completely.
2. Solder usb input directly to battery terminals bypassing charging circuit.
3. Connect portable power bank to miniUSB port.
This would give real and constant 5V, but I'm not bothered with doing so, I'll stick with component swap and tweaking as I think there is more potential there.
More to come when I will get rest of the parts!