The fact that conventional multi-way speakers (CMW speakers, for short) are a bad match for single-ended tube (SET) amplifiers is a common knowledge. Speaker sensitivity is most widely considered to be the main reason for such mismatch. We present an additional explanation, namely that the CMW speakers distort SET amplifier frequency response, and may be responsible for SET amplifier undeserved reputation for weak or sloppy bass and rolled highs. We demonstrate that highly reactive nature of impedance and impedance swings of these speakers is the cause of this effect.
Conventional multi-way (CMW) speakers usually contain 2 or 3 bands of drivers connected to passive crossovers separating signal into different frequency bands. It is a known fact that impedance of such speakers varies greatly across the audio range. Consider the following equivalent schematic for a reactive load closely approximating a conventional 2-way speaker:
This load exhibits a response with characteristic spikes in impedance. We obtained this data with PSPICE simulation (student/eval version):
3.   Conventional configurations
Conventional multi-way speakers are best driven by powerful solid-state (SS) amplifiers. Output power is usually not a problem (i.e. SS power is cheap) and one consequence of this is low sensitivity of CMW speakers. With abundance of power, it is possible to achieve good linearity from relatively inexpensive drivers by dampening and filtering. SET amplifiers usually badly drive such speakers and the main reason considered to be sensitivity mismatch: a <10Wt SET amplifier can not drive a <86Db/W/M speaker loud enough. Unlike that, SS amplifiers play CMW loud and measurments show good frequence data.
Another characteristic property of SS amplifiers is very low output impedance. How low? A small portion of speaker's impedance, and usually well under 1 ohm, frequently in .01-.1 Ohm range. Thus we can describe SS power amplifiers as voltage sources.
4.   SET Amplifiers
Let's assume sensitivity mesimatch is not a problem - either we have a very powerful SET amplifier, or we listen at low levels. We'll start listening, delicate sound. We'll be likely unpleasantly surprised with muddy or boomy sound which is also could be noticeably rolled off at both limits of the range. What is the reason? We loaded a 2-stage feedback-free SET amplifier with a 8 Ohm CMW:
PSICE simulation produced the following voltage-vs-frequency curve:
Explanation? Output impedance of a SET amplifier. Output impedance of almost any SS amplifier, due to deep NFB, is extremely low, well under 1 Ohm, sometimes as low as .01 Ohm. This makes severe variations in load impedance virtually unnoticeable. Unlike that, a typical SET amplifier with no NFB hase an output impedance in 4...10 Ohm range.
The above curve is with a 8 Ohm load. Many CMW speaker are 4 Ohms. Teh resuts is wortse:
The moral of this story is that SET amps, and in particular those with no feedback should be coupled with speakers that are "easy loads", Feeding signal from such SETs into speakers designed for amps with extremely low output impedance does not make sense because such speakers usually distort SETs responce. Of course, finding speakers that are SET-friendly is not easy. High sensitivity is usually the most sought-after quality but low reactance is another one.