240 Volts is 240 Volts, whether it's from the wall socket or from a transformer. As long as the transformer has a high enough power rating for the amp you should be fine. I'm going to err on the side of caution and say if in doubt, get a tech to look it over. If you know the amount of current your amplifier draws at 240V then just make sure the VA rating of the transformer is sufficient for the job.
this doesnt sound right to me. maybe im misunderstanding.
a US amp will generally run on 120volts, even though it could run on a few voltages, that might not be wired in to a switch for example. in which case you need a 'step down' transformer to shift 240volts down to 120volts.
running 240volts into a 120volt amp doesnt work. ive accidentally knocked the voltage switch on my marshall before to 120volts and then plugged in 240. it evaporated the mains fuse.
HTH is on about replacing the transformer with one for EU voltages.