The SEX is both a speaker and a headphone amp, and as a speaker amp, it's going to give you a very low noise floor, but not a lot of power. This amp also certainly offers the fewest tube rolling options, which will drive some people nuts.
The Stereomour will sound quite a bit better than the SEX amp, being a DHT amp. Its noise floor is a bit higher, as it uses AC on the filaments, but it gives you more power and several inputs. Because of its design, the Stereomour can clip quite hard on transient peaks without bothering the listener. Tube rolling options are incredibly broad, though rolling 2A3's can get expensive.
The Smash/Seductor gives about the same power as the Stereomour, but with more gain and more flexibility for upgrades. The Seductor won't quite have the DHT sonic purity of the Stereomour, but it will work better with many modern speakers that have better bass response from an amp with more damping than a Stereomour or SEX amp. The Seductor also offers a low noise floor. Clipping performance with the Seductor is somewhat marginal, after about 6 Watts, the triode distortion spectrum slips away and distortion becomes apparent. Tube rolling options for the Seductor are vast (not so much for the Smash).