Leemincy:
An amplifier's output impedence "reacts" with a speaker's impedence, and vice-versa. Many tube amps have a generally high output impedence that features large peaks and dips (solid-state amps tend to be more linear in this respect). The impedence of speakers can also vary widely and feature peaks and dips (with cone speakers, usually in the bass, but not always). The uneven nature of a tube amp's output impedence can cause a speaker to have uneven output, especially where the speaker itself has a roller coaster impedence, which is to say, the lows, mids or highs do not all put out the same volume in response to the same signal volume. For example (and this is purely a hypothetical), in response to warble tones that should result in 90 db. of sound coming from all frequencies, the bass and mids would put out 90 db. each, but the treble would put out 95 db. Thus, on music, the treble will be too strong relative to the other octaves and will be perceived as sounding hot or screeching. If it is the bass that is exaggerated, the speaker will sound boomy. When John Atkinson measured the Nova Utopia Be for Stereophile, he wrote:
"[D]espite its having a nominal value of 8 ohms, the Nova's impedance varied considerably, remaining below 6 ohms for much of the midrange and above 9 ohms for much of the treble (fig.1, solid trace). If the Nova Utopia is used with an amplifier having a highish source impedancea classic tube design, for examplethe treble region will shelve up by a couple of dB, changing the perceived balance considerably. However, it is probably not a good idea to use a tube amplifier with this speaker, as it is a demanding load in the bass region. Not only does the impedance drop almost to 3 ohms in the midbass and the middle of the midrange, there is a punishing combination of 4.1 ohms magnitude and -50 degrees electrical phase angle at 62Hz. A good 4-ohm-capable solid-state amplifier will work best with this speaker."
I hope this responds to your question.
