There's no comparison on youtube between the Ashdown Studio 15 and the Fender Rumble 200, there are few sound clips of the AS15 and -- somewhat surprisingly -- also few of the FR200.
However, there's a Thomann video that compares, among others the AS12 and FR100.
It would seem that the AS12 is not as loud as the FR100 (although the volume knobs aren't quite in the same position), which may mean that the Ashdown speakers have a lower sensitivity (and neither can keep up with a drummer unless mic'ed).
However, the AS15 is 300W vs the 140W of the FR200, which should make up for a 3db eventual difference in speaker's sensitivity.
Vs the AS12, the AS15 should be more than 4dB louder, so can probably keep up with a drummer.