What an amp simulation is doing and how you can do that without
As this also goes for most other amp simulator or any other special audio software being sold for achieving a certain sound simulation from original gear in your audio signal chain, this article can be seen as a prominent example regarding audio software industry marketing grey boxes unmystified by an audio professional. Why do I limit the "how you can do it without" to simulation software and do not take the real gear counterparts in it as well? Well, because of the (not so) "little" difference they still make inside. And the reasons why (and so by what) they have been build. The most newer comparisions out there in the net often forget to bring up the most important part in the question regarding how close simulations can get: the fact that the original gear has been build from real parts for their sturdiness and real practical need in the studio and on stage in a time where sound was created with a lot of electricity. And most of the original gear run thru complete different parts to create that sound than the counterparts do in software. You can compare how close they get, yes. But you can not compare how it has been achieved. That are two complete different stories. With some exceptions of "real" gear nowadays which finally now also runs software in the box. But this is a complete other story... (For those who don't know what an amp simulator/emulator is: https://en.wikipedia.org/wiki/Amplifier_modeling)
This story here goes for bits, bytes and code which runs on a code interpreting and processing machine to simulate sound modifikation in the so called "plugins", commonly and widely used in so called DAWs on a studio computer. And if you focus on this as a real audio professional and somebody who maybe even developed own audio software, you will early realize and understand that it finally all runs thru the same - let's simplified say - "groups of computer code blocks", grouped in dynamic range modifiers working on the steps of audio bits, wave synth additions, noise and dithering algorythms, frequency modifiers, etc, etc. Just the housing changes. Of course VERY simplified saying...
Guests in my studios know me often saying that I would prefer that plugins would more concentrate on the quality of the sound modifikation and the real usability and automation options of their faders and knobs than on their fancy GUIs simulating the original gear in HD size. They should rather encourage the user to listen not to watch. But this is how it sadly has turned into over the decades. People always love to believe in miracles. So when they see an animation of an audio tube-alike looking plugin orange blinking on screen, they believe that their sound has changed to a real tube saturated sound. In fact, jsut a chain of the already mentioned code blocks are put in order to "simulate" a tube saturation. And the quality of the simulation is doubtful...
Same happend with computers as a whole. Read on about it from original computer legends who always blamed the industry for changing the thinking regarding computers already in the beginning of the computer century. Where afforts have started to put more into the "realistic" look and feel on screen philosophy to sell more computers instead of supporting the thinking into the more sense making direction: endless flexibilty and functionality by code blocks. The most important difference of computers in comparision to their real counterparts: In computers both - the realistic looking user interface (GUI) and most possible flexibility - both can not live side by side in machines run by code. Because the GUI will always limit the underlying functionality. You don't beleave me? Think about it, read about it, think about it twice and you will learn it. After your conclusion you will never start any fight regarding PC vs Mac vs Linux again in your life and you will look at your kitchen machines in a complete different way. Because you would have understood that it doesn't matter after all. As I often stated: we fly to the moon but we are not able to develope stable printer drivers. Why? Well because the computers carrying rockets flying to the moon have a very minimalistic GUI and are concentrated on stability while the installer GUI for the printer driver gets even more attention than the printer driver itself. But hey, it looks fancy while it installs the crap, doesn't it? :-)
When I wrote let's simplified say "groups of computer code", well, of course it isn't that simple. Many thinks can change the final result here. The order of the signal changing modules, the carefulness regarding signal integrity and its resolution, the understanding of how digital audio data has to be treated in bits and bytes and how it finally turns out when A/D D/A convertion goes back to real audio signals.
To bring it to the point behind all that: yes, of course, you still have to be or have a real audio physics professional watching over a good audio plugin development when you are serious about it. But the sadly truth is: most of the plugin boutiques do not have one, or at least not enough of them for all their products in the house. And this is where my leading headlines comes back into the story. If it is about to simply copy one of all the mass products out there simulating a certain "sound" in the chain like an amp is doing, try to understand what an amp is really doing, then think about which computer modules which already run in other plugins will be part of it and you will quickly come to the conclusion that you can do it with 3,4 other plugins in the right order wihout the CPU hog of a amp simulation plugin.
Of course this is NOT true for some real smart plugins developed and watched by real audio physics professionals, but this is out of price range for the most plugin users I think.
For those who need some help start thinking about what kind of plugins could replace an amp simulator, simply check out how an amp is build and let's make some parts clear. The line-up could become endless depending on what amp you would like to simulate. But most amps definitely have a saturation module and a non-phase linear or rather say harmonic equalizer, plus a gain clipper, special type of compressor and kindof bit crusher in it. Placing them in different order and listen carefully by setting them up you will surprisingly find out that it can hold with most amp emulation plugins. Plus the advantage that you really know which parts are causing which detail and can go deeper into it to change things.