Your combo won't be putting out the full 575 watts into one speaker (15 inch I assume). You'd only get the full wattage with two 8 ohm cabs connected to the amp giving a combined impedence of 4 ohms. The old speaker will, more than likely, have been 8 ohms which means the amp would only have been putting out about 350 watts. A 400 watt 8 ohm Celestion or emminence speaker would be more than capable of replacing the old Ashdown speaker.
MB1.
+ Twelvety...listen to what the man said!
Ashdown will have a replacement speaker, and you wont affect the combos resale if you replace it like for like!
Have you got a blue one?.....Fnarr Fnarr!,