i don't know how it would sound with the fractal amp (if that's what you're actually asking)
...but i gotta say, i have the ISP vectorSL running it live with a mesa 4x12 with the band, and there's no way i could play without it! (i did, when i didn't feel like lugging the thing around to small gigs...but it never sounded right). playing next to the other guitarist without one made his tone sound, well...never as good, we'll just say. and the drummer/ bassist would usually make a comment about the "thin" tone when i wasn't there for a practice (even tho he did have a good setup mesa thru a 4x12 marshall).
though this is just a personal preference.
but this really depends on the tone/ music you play...i really don't see it working well unless you're playing 7+ strings for that tight low bottom. --but i could be wrong.
i've been hoping for someone with a good recording setup (something i don't have/ have good access to) to make a good set of IRs with a vector SL and some cabs. but to be honest, i'm not sure if it'll work in the IR world or not...but would LOVE to try it.
i just think back to when the pod was "the" tone everyone wanted...most used the bigBottom in their patches when it came out--the randall cab you mentioned. and it really made the tone shine (for "that type" of sound that is).
i have been messing around lately with using clawfinger's krank porthole IR [thanx clawfinger!] to "mimic" the sub with redwirez orange cab IR i'm using as my "default." from the amp i split the signal into 2 different pEqs/ filters, one loPass and one hiPass at 130 then each into the specific hiRez cab block. then both of those blocks into a mixer so i can dial in the cabs quickly. it's not the same tone/ sound as the sub live, but it's working out as a way to add in some of that "feel" for direct recording.
...i'm not sure that you'd get any benefit using a sub with the fractal amp as that is supposed to be a flat response and as a stand alone, really what you should be hearing in the first place...i think the sub works best with a big guitar cab (imo).
so: no...not necessary.
but i love mine--with my 4x12.