I rewired my B4r myself, it was a very easy job. Just a case of moving a few crimp connectors. I did have an ampeg technician to tell me which ones to move to where though.
The point is that I didn't have to use another transformer, basically by me rewiring it in parallel instead of series (or vica versa) it took 240v instead of 110v.
Don't have a clue about the Pro 6. But you never know.
Step down transformers would need to be rated at more than the wattage of the amp itself, so with the amp in question you're talking at least 1100VA. And they aren't exactly cheap. It'll get very hot as well.
Edit: Muppet it cost you £120! Jeesus, that's alot for labour. I changed the circuit breaker and a few fuses as advised, parts totaled around £8 or something. It was then a 5 minute job to swap the connections. I wouldn't do it if the amp had a warranty, but if you're importing the amp you're not going to have a warranty anyway.