When you put a speaker in a cabinet, it changes this peak and makes the speaker have a different impedance curve. Sealed cabinets tend to shift the peak up in frequency. Reflex cabinets create two peaks, both having less impedance than the single free-air peak. Horns and transmission lines create multiple peaks, generally lower in amplitude than the free-air peak, depending on flare shape and rear chamber.

Impedance fluxuations cause problems for tube amps, altering their response curve. The speaker impedance forms a voltage divider with the amp's output impedance, and if the ratio of impedances fluxuates, then the power transfer does too. So to make a long story short, if you have a small signal tube amp, you'll want to find a speaker with a low impedance peak at resonance, somtimes referred to as "Zmax."