Many say tube sound is nothing more than EQ and designer coloration, if this is the case would it be possible to exactly replicate tube sound through the use of a plugin?
Lets stay with current state of tech and common understanding of the topic.
Those who typically tend to believe in measurements and typically has a technical view on sound reproduction, argue that the sound made by tubes, is distorted. This basically stems from the fact, that this is what is measured. Using the positivists view, this is usually proven with amplitude charts, and some rare instances of more complex waveform tests. It is not really understood why these measurements do not correlate with the listening experience people report of having. If you move along this paradigm, it should be possible to make a filter that distorts the sound, that would reproduce the tube effect.
The main other school of thought, are those who trust their hearing, rather than the measurements. People in this category, in which I tend to be, do not get the same experience with any of these tube filters. It is simply not what our ears tells us.
Let me just add, that I have no idea why there is a huge difference in what the theory tells me should be the case, and what my experience is. The theory about the measurements appear sound as, well, something. Yet my experience is in something completely different.
My guess is that sometime in the future, this will be better understood. When that lack of understanding is bridged, we probably should be able to produce a single piece of gear, that would be able to reproduce current solid state and tube rendering. My guess is that by then, both renderings would need some form of filter, as the current rendering of any gear, simply is not perfect.
The main thing about tube rendering, is the speed. The perceived speed. It is also more raw, as in a bit lacking in precision. This gives it a raw feel, and tend to add some life to the music. It is a loved rendering for guitars. The randomness is nor easily generated by any equalizer, as there is no randomness to it. Also, the ground is actually floating in many tube designs, which tend to add effects that are complex. My guess is that those just thinking this is a amplitude thing, hits the mark as lack of technical understanding of the innards of the tube design. Also, many tube design are sensitive to interference. In my experience, extremely sensitive. Designing a filter to mimic that, would require deep insight into the innards and functionality of tube designs. It probably is nothing but simple. The truth is that all this is just speculation, as it would be for anyone else, as we do not understand this, and certainly do not know.
What you probably could do, with relatively ease, is mimic the amplitude curve of a know tube amplifier. There probably are some other tricks that seem to make sense as well, I just have no detailed insight into the current state of the matter. However, you would find a lot of people who claim that such filters do not sound like the tube rendering it is supposed to mimic.
In the photographic world, there is something called and "unsharpen mask filter": It actually reduces sharpness, but for us humans, the image appear sharper. My guess is that there is something like this going on, just that in the case of audio, this is simply not understood yet. You would still find people claiming that a signal converted back to analogue using a DAC, then converted to a vinyl record, then read with a pickup, fed through a RIAA amp, then fed into a system, actually improves the sound quality. Even if you use the exact same DAC, and feed it directly into the amp, skipping the vinyl loop. Why all those vinyl stages should contribute to any accuracy, is really hard to grasp, yet people seem to have just that experience. These are the same people who tend to love tube amps. Just more of the same thing.
Both camps are probably right, given how they argue. There is just a bridge lacking. A bridge of understanding.