There is no way to disable the preamp of an amp model (as much as some of us would like that...) but there is a workaround I use when I want to use a preamp pedal or other rack preamp.
I use the ‘tube pre’ model, then turn on power amp modelling (turn sag up above 0), and copy the speaker page resonances off some other amp model.
The ‘preamp’ section of the ‘Tube Pre’ model is supposed to be neutral, and the eq is flat when the controls are all at noon. Presumably this isn’t really any different than the front end of a rack power amp, or the FX return buffer in a tube amp head. Keep an eye on your input level and the input drive of the model.
You can also copy the advanced parameters out of another model if you like. I’ve spent a while tweaking one patch to sound similar to my Fryette 2/50/2.
Some parameters don’t act like they do in other regular amp models, or don’t work at all. For instance, the dynamic depth/presence controls don’t do anything.
If you don’t want to go this route, there are other hardware units out there that model power amp only, such as the Amplifire, Mooer Radar, Torpedo CAB. I’ve never tried any of these though so I can’t compare.
