I am looking for a 230 to 110Vac converter / tranformer for these...
I know this is a bit off topic and slightly pedantic, but it always puzzles me when I see this. You appear to live in the UK where the standard supply voltage is 240 V AC, 50 Hz. You are looking at a product for the USA market where the standard supply voltage is 120 V AC, 60 Hz. So why 230 V to 110 V? You would be better off looking for a 240 V to 120 V transformer (if indeed you need one at all).
Availability? 110VAC is a very common voltage on building sites, in the UK and transformers are widely available.
To the power question, amplifiers have an efficiency and may produce some waste heat. So the input power will be slightly higher than the output power. On the other hand, you are unlikely to be running the speakers at full volume, so they will not be running at full power. On the third hand, it is always good to have an operating margin on the power supply. So probably go with 300 W or more power rating on the transformer.
Manufacturers also exaggerate the power output of their amplifiers, so it's a tricky one.
If the amplifier really is rated to 120V RMS, then I'd probably suggest a similar sized transformer, since the average power output, even at high volumes, will probably be around
1/
8 of the maximum and the transformer will be able to withstand the surges.