Filters. Yup, multiple parametric/high cut-low cut/shelving stages. A cab based on filters doesn't have that "digital" sonic veneer, takes up a lot less CPU power than convolution cabs, and just shifting a few parameters around makes for an entirely different sound. The vibe also seems to be "bigger" and somehow thicker.
I've been putting noise through amp sim cabs, checking the spectral response, and then duplicating the curve as closely as possible with EQ. Although the sound isn't as "nuanced" as digital, in the sense of having thousands of stages instead of a half-dozen or so, you can still hear that a 4 x 10 cab sounds a lot different than a 2 x 12.
This also makes bi-amping and multiband much more practical, because you can pile on the amps without bringing your computer to its knees. Fun stuff!!!