Guys consider this, the main function of capacitors is to remove voltage fluctuations and electrical signal noise.
The main electricity is AC at 50-60Hz, but all of the electronics and motors in the printers is DC. Capacitors play an intergal role in rectifying AC to DC.
However, data signals are essentially AC, and capacitors play a significant role in ensuring signal integrity and supppressing transient DC volatage on signal lines.
Well designed and built electronics has a margin for error / overhead with respect to the components as well as circuit board design to maximise signal integrity and performance.
Your printers are built up to a standard, not down to a price.
Capacitors have a service life, no matter what grade of components are used initially.
If you are unable to print anyway, my suggestion would be to find a local TV repair man, one that actually repairs them and have them replace the capacitors on all of the boards.
Or if you want to minimise the cost, trace the signal path(s) from the main board to the slider board and the heads and have those replaced. BTW get the slider board caps check/replaced also.
They will likely have an ESR meter, which measures "electrical series resistance" in capacitors and may have be able to also measure the capacitance to check the actual values.
Now this is an educated hypothetical supposition on my part, but if the large dot -> 360 DPI is printing clean though coarse, the larger signal (higher voltage, longer duration or both) has a better signal to noise ratio.
peace out