Well, you get into variances of the B-15N to the B-15S of the 70s, too, because a B-15 isn't just a B-15. We know this, but people tend to associate it as a generic thing. The bottom line to me having owned a lot of Ampegs over the years is that even going through another cab, you still get that 'Ampeg sound' as I like to refer to it. Even when I ran the SVP-CL through the Berg IP112, you can tell it's Ampeg.
So you need to think of the sound as a more complex thing than just the cab and speaker - which do play in - but at the core of it is that Ampeg sound people know and love. But you can still run a B-15 head into an 8ohm cab and get some of that sound, but you're not getting the matched cab. THAT's the difference.
I still find the VT Pedal to be too sterile compared to something like the Linden EQ + TruVoice. It's missing something that combo, as well as the B-15 (any of 'em!), have.