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MOSFETS are most suitable for vacuum tube еmulation, because similarly they convert voltage to voltage, whereas BJT's convert current to voltage. The major difference between MOSFETS and tubes is that tubes work with negative grid in respect to the cathode and MOSFETs work with positive gate in respect to the source. There's also the difference for the gain but it can be properly corrected if we adjust the parameters of the MOSFET appropriately. One more difference is that you can't choose an autobias for the MOSFET. It's always fixed bias, because MOSFETs don't have the initial conductance like tubes do, to establish the autobias regime. Some other differences are the V-I curves and more precisely the more sharp cutoff of transistors, which is actually what causes the distinctively warmer sound of tubes. See transistors flatten the wave when in cutoff, leading to the creation of a odd harmonics, which are unpleasant to the ear. On the other hand most tubes have a more smooth curves leading to a rounding of the waveform when in cutoff, instead of the sharp flattening of the transistors. This in turn creates mostly even order harmonics, which up to a point, are not unpleasant to the ear, on the contrary they enrich the sound, making it sound better at ,,attack'' moments.
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