Yes I do, and have owned a bunch of tube amps as well, which I have confirmed to perform no better than a great modeler when running into IRs and studio monitors.
So I'm more convinced that the issue is not the FM3, but user error. So I recommend you explore if you can get it to your liking rather than jumping into buying an old preamp which has its own quirks and potentially might require repairs.
Connect the Kraken "Through" output to the FM3 Input 1.
Connect the Kraken "Loop Out" to FM3 Input 2.
Make a preset that looks like this:
View attachment 17195
Now by turning the Kraken on/off, when off it should go through to IN2 -> Amp -> Cab -> OUT1 and otherwise just through the cab.
Start by using whatever IR you like with the Kraken so you rule out the cab sim.
Make sure your output levels match - the Kraken should not be louder than the Amp route.
Then try to figure out an amp model that would be closest to your Kraken. Since it's not modeled on Fractal, I'm guessing that maybe one of the Friedman or Bogner models might get you in the ballpark.
Now you can compare and try to figure out where it's going sideways for you.