According to (some) tube afficinados, you can simulate pre-amplifier can emulate a tube pre-amplifier: small-signal elements based on tubes can be substituted with silicon. However, you probably can't emulate that big signal stuff that goes on between the tube power amp, its output transformer and a speaker, and the sound that ultimately emerges. That is to say, not in the same amount of wattage, and with the same speaker. Maybe a 5,000 watt rig (amplifier and speakers) could faithfully emulate what comes out of a fully cranked 50W tube amplifier, its output transformer, and a specific 50W loud speaker.
As a related illustration of the idea, it is common to see microphones in front of small tube amplifiers, which then feed into a large PA system. The small amplifier acts as the "signal processor" which produces the tone, and the PA just makes it bigger. It has the headroom to do that.
In that scenario, the tube amplifier with a microphone in front of it can be replaced by digital modeling: something which produces the same transfer function as what occurs between the signal from the instrument, and the signal going into the console's microphone input.