Don't know if you're asking about models not in the FM9 or how to get the FM9's model of an amp you own to sound the same. To answer your question from the latter position, I have never obsessed over getting an amp or effect to sound 'as close as possible'. I have been able to replicate several amps and pedals to my satisfaction but never got to the point of being hyper-critical about the differences, if any.
I have yet to find an amp model in the FAS system that doesn't provide exactly what I expected from the physical amp. The ability to adjust and tweak so many parameters that really aren't accessible on a physical amp tends to create a lot of angst and desire to make an amp 'perfect'.
If you were to buy a second or third physical amp of the same model, how much would you obsess over making sure they sounded 'as close as possible'? There's been a few threads discussing the differences in component tolerances that makes is extremely difficult to have two amps of the same model sound exactly alike, side by side. Would you measure parts and components and attempt to replace them with others with closer tolerances?
This is a rabbit hole with no bottom that, from what I've seen, only leads to a lot of frustration. Accept the FM9 for what it is, a collection of some of the best amps on the planet accurately recreated in the digital world. You have the option to obsess over minute adjustments, if that's what you like to do, go for it. If you want to play guitar, pick an amp and tweak it as you would your physical amp and enjoy.