With a tube amp, the issue is not only whether or not the amp can drive the low impedance load. Because tube amps often have a high source impedance, the speaker impedance at all frequencies must be high enough so that frequency response is not affected significantly by the high source impedance. Also, because the speaker voice coil is moving in the magnetic field or the driver, it develops a "back EMF" that can be absorbed (damped) by the amplifier if there is a sufficiently high ratio between the speaker impedance and the amplifier impedance. Tube amplifiers, because of their high impedance provide less damping and this becomes significant if the speaker impedance is too low. The extent to which such "ringing" or lack of damping is deleterious to the sound is a matter of taste. This is why tube amp fans generally prefer their speakers to have a high impedance.
I tend to find that, while there are theoretical reasons for avoiding low impedance speakers for tube amplifiers, in practice, even 4 ohm speakers can be mated with suitable tube amps. But, I wouldn't push it to such an extreme as a speaker with a nominal 2 ohm impedance, unless I have an opportunity to audition the pairing.
I tend to find that, while there are theoretical reasons for avoiding low impedance speakers for tube amplifiers, in practice, even 4 ohm speakers can be mated with suitable tube amps. But, I wouldn't push it to such an extreme as a speaker with a nominal 2 ohm impedance, unless I have an opportunity to audition the pairing.