Fractal in a different thread talked about why a tube amp sounds/feels than a solid state amp given the fact a tube amp has less wattage. If you look up the topic of power amps I believe you'll find it.
I have been using amp modelers and tube amps since the beginning like many people have, however that doesn't mean I know a dang thing. What I think I can say is I've used many different power amps with modelers and types of speakers. I still own both as well. I may be ignorant here , but the term "not true to the original" comment puts me off a little. Look you go by what sounds best to you , for your situation, or whatever metric you use to determine what you need want and like. As soon as you plug your guitar in its not true to the original, your not using the same pickups, speakers, guitar, room, amp settings and so on.
Fractal made their units to obviously work with any powered solution you apply. They made theirs from the beginning to react as close as they could get it to the original device they are modeling, but added ways to equalize room issues or other variables in their amp model being slightly different than the real one. They even gave us the ability to change the amp internals and make it a hybrid.
We also have to keep in mind no two amps or speakers sound the same, as I had bought my cabs in pairs. I can play my Axe fx iii through 4 different power amps and get 4 different flavors, but they aren't really all that different. In a mix you wouldn't be able to tell, but if im in the room alone yeah the difference is easier to hear. It is easier to distinguish if you are the one playing the guitar and not just listening to the person playing.
With the tube power amps and with minimal adjustment I could get them to sound extremely close close anyway, but why should we car about that part. They all still would sound like the original( my virtual amp) in my room anyway. I'm not a fanboy just someone who likes a good product purchase. I say buy a few different ways to listen to your modeler. If you can afford to keep multiple options do it, as we all know sometimes we just need something a little different. For me fractal is at a point where at least on my virtual amps all I have to do is make slight basic adjustments to the amp eq section and your pretty much their, which is absolutely crazy the tech is at this point. I'm still debating why I have my tube amps. I believe I'm just gonna keep the tube power amps and sell all of my virtual amps except one.
Waiting on Fractals new touch screen unit, yup their gonna do it because someone else has