When you select a block, then push the Action button, then use the toggle to 'drag' the selected block from its path...thus creating a parallel path to the original location...then push Action again to exit the Action mode, a "Spilt" and a "Merge Mixer" are automatically created to accommodate the creation of the new path.
Are you using the full amp models or the amp "pre" models?
If it is the full, try the "pre" versions in stead since your combo amp is providing the power amp model and the real speaker...
And when using the pre versions you still can select a cab on the amp page, it doesn't give you the same effect as the full amp with a cab which can be kind of dull when playing through a real power amp and guitar speaker/cab but it does give a bit of an EQ curve to the output to give the sound a touch of the flavor of whichever cab you selected.
So, for example, A Blackface 'Lux Vib "pre" will sound a bit more like the Fender it is modeled after if you also select the 1x12 Blackface 'Lux cab.
And instead of using the loop output try using the regular output and in the set up menu select Combo Power Amp as the output setting (on page 4/10 of the set up menu).
And the most powerful tone shaping tool that is often overlooked: EQ! Put a studio eq just after the amp block and dial in a touch what ever frequency (or dial out) that might be too weak (or too strong)...experiment a bit and you will find you can really bring some life into it.