The cab block loads an IR, that's just frequency filtering, its not connected to the amp block.
The new speaker selection in the amp is related to the power amp, how different cab/speaker configurations make a power amp respond, it's dynamic, with a little bit of frequency response influence.
The ideal combination is using an IR of the same type as the speaker selection, then you have power amp resonances matching up with the IR frequency response as it was shot...but you can use the same IR you were using before, and still change the speaker type to a 1x12 or a vox 2x12 if you want, and all that will happen is you will hear the power amp start putting out a little different low end resonance, get more focused...it's just a power amp preset to simulate having different cabinet types plugged into the amp. Your favorite IRs will still sound like your favorite IRs no matter what speaker preset you use, it's just now if you use the speaker preset for the same cab type as the IR, the resonances will match up like real world and sound a m a a a a a a z i n g