Aw gee, I hate to ruin a good "Let's bash Ampeg for no good reason" thread, but...
The SVT 15e goes down to 33 hz and tops out at 3k. The SVT 410he goes down to 43 hz and tops out at 18k if you use the tweeter, 5k if you don't. Now a more elegant solution IMHO is to use the SVT 410hlf if super low end with super highs is what you like, but this is another option, and it's an option using sealed cabs vs the 410hlf, which is ported. And while I don't mind using ported cabs at all, there are many bass players who do. Now I don't care for this combination and I prefer matching cabs, but a 410 + 115 is very often used by bass players. And in speaking with them, many of them love their unmatched cab rigs, too. This has not escaped the attention of many amp companies' marketing departments, and many companies play into it. Ampeg is by far not the only one who does.
As for the potential phasing issues, yes, there can be some when mixing cabs in this fashion, but there are also some companies that go out of their way to make sure that certain combinations of their cabs keep these issues to a minimum, so it's not a given that all mixed cab rigs will work poorly together.
Again, I'm not agreeing with them that this is an ideal combination. I'm very much on record saying that I prefer matched cab rigs, output of a 410 vs a 115 is usually unbalanced, and random pairing of a 410 and 115 because you assume that the 115 will go lower and the 410 goes higher is listening with your eyes. But in the case of the SVT 410 paired with the SVT 15e, it actually does work like that. Substitute the SVT 410hlf for the 410he, however, and it no longer works like that.
So by all means, feel free to continue the bashfest, but try to remember two things:
1. A lot of bass players use similar combinations and love them.
2. Nobody is holding a gun to your head and forcing you to buy a rig you don't like.
3. Knowledge is power, but as I've learned over the years, incomplete knowledge is incomplete.