Interestingly, you get some extra impulses to run alongside certain speakers to more closely replicate the reaction of that speaker with a tube amp. So you're running two impulses at once. Just out of interest, can the Axe FX run two simultaneous external impulses?
It adds a bump in the lows at the speaker's resonance point, scoops out the mids and then adds presence and a lot of air. A tube amp will do this because, compared to a SS amp, it has a relatively high output impedance which effects the speaker's frequency response and starts "bending" it to look more like it's impedance curve.