How will I know if my equipment can withstand the fluctuations, unless it breaks, then I know it can't. Also, then I cannot risk buying used from the EU which has a 220v
Surely you have to carry the proper research.
All EU, including the UK has 230V. However in mainland it is +6/-10% and the UK +10%/ -6%. Since then appliance manufacturers have been testing at 230V +/-10%.
Imported equipment usually works at twice the voltage limits of US - the transformer primaries are just connected in series, not in parallel.
Most equipment has regulated DC supplies. However many times the tube filament power supplies are AC not regulated and sound quality decreases significantly when the tube cathodes are insufficiently heated by low mains voltage.