Despite forming a good part of the synergies making up the sound a player can have, often amps (together with cables) are literally the poor relation in the signal chain. As the old saying has it, a mediocre guitar through a good amp will usually sound much better than a fab guitar through a rubbish one. Though you may be of course intentionally be going for that lo-fi garage sound… it is all subjective!
Amp work divides naturally in to different categories:
Repair – where the gear has stopped working or doesn’t function correctly and needs to be got up and running.
Maintenance – amps do need periodic maintenance or their sound will deteriorate over time.
Restoration – work to return an amp to something approaching original condition and/or sound (including possibly to undo previous modification).
Modification and upgrade – instrument amps are very tweakable, and even quite small changes can dramatically alter their sound or voicing, and changing speakers can also have a big impact.