Actually, that's a great question.
The answer is you can get very similar sounds out of both depending on your settings. With knobs at noon the B-15 is brighter, however, your knobs won't match up but you can EQ them to sound quite similar. The 810e cab is also a little brighter than your typical B-15 cab, but they both have that Ampeg thing going on. And of course the SVT will get way louder and stay clean at a much higher volume, but I think you knew that