OTL amps tend to have a high output impedance. That means that, if they are matched with low impedance speakers, particularly ones with an impedance curve with wide swings up and down, the frequency response of the speaker could vary quite a bit. The ideal speaker for all tube amps, particularly OTLs and SETs (which also tend to have highish output impedance), would be one with a high nominal impedance and a curve that is relatively flat.
Theory aside, all speakers require careful matching of amps. I haven't noticed, in the setups I've heard using OTLs, any particular sensitivity of the OTL type; they tend to sound very good with a wide variety of speakers.
Theory aside, all speakers require careful matching of amps. I haven't noticed, in the setups I've heard using OTLs, any particular sensitivity of the OTL type; they tend to sound very good with a wide variety of speakers.