2. A 25 amp breaker should be minimally sufficient, but remember the breaker is there to protect the branch circuit wiring and not the appliance. The amplifier should have internal fusing to protect itself.
3. Consult a wiring table in the NEC handbook or an equally qualified data source in order to select the minimum wire gauge consistent with your branch circuit wiring run and acceptable voltage drop. No smaller than #12 for even the shortest run.
Always put the burden on the breaker NOT the wire, the NEC rules are there to protect you from fire.
Just read Don's post, totally agree, if the run is long or the 10 ga is a much better choice but I still say 20amp breaker, the breaker should be the first to go...
I, and my electrician, believe in a margin of safety.
The PG-XL uses C13/C14 sockets/plugs on the amp side. I believe these are rated at 15A. Since the amp will be the only appliance connected to our 240V circuit, at one time I considered replacing my 20A breaker with a 15A for that reason. But I decided to stay with the 20A since (as has been noted here) the amp should have its own internal protection.
Some of the pictures on the 4O3A site are renderings I believe. Ria confirmed that PG-XL uses a C13 connector:
and said that she uses this cord:
which has C13 and 6-20P connectors.