Well in theory, the Ashdown will be putting about 180 watts into the 8ohms internal speaker, so it should be louder than the Rumble. But they have different amp sections so I couldnt say for certain the Ashdown would be louder.
I havent played the Fender but have used a Mag 300 head for 10 years, a lot of the time through a 15 inch speaker and it has been loud enough for me when doing large pubs, playing rock music.
Also, if you add an extension speaker, say a 2x10, then the Mag will give you the full 300 watts. I run my Mag into 2, 1x12 Mi cabs and there is plenty of volume.