We often hear how one amplifier is a good match for one speaker, a poor match for another. Why is this? What makes the association succeed or fail? Let’s look into the electrical relationship between the speaker and the amplifier to learn more about the interactions that occur.
The Loudspeaker -
What makes one speaker an easy load and another hard to drive? There are a number of answers, but the two most important are impedance and sensitivity. First, impedance. Impedance is resistance that varies with frequency. The fact that the speakers’ impedance varies with the frequency it is reproducing, is one reason why the amplifier has such a tough job.
For example, let’s examine a speaker with a nominal rating of 8 Ohms. The term nominal, means average, thus is not only possible, but quite likely that significant impedance variations will occur throughout the speaker's frequency range. Indeed, a variation as wide as 3 to 40 Ohms is possible. The amount of variation, in addition to how low or high the range, determines how difficult the speaker is drive, and thus defines the amplifiers role. The amplifier must be able to deal with these impedance variations, producing the amount of power necessary to drive the speaker at any frequency. If the amplifier is not capable of dealing with the impedance swings, audible distortion occurs.
The Amplifier -
Ideally an amplifier should be a constant voltage source. That is, for a given input signal, the amplifier should produce a constant voltage across the speaker terminals whatever the load. For example, if the amplifier is producing 20 Volts at the output terminals, Ohms law (R=V/I) tells us that there are 50 watts being fed into an 8 Ohm speaker (watts equal voltage squared divided by impedance). If we connect a 4 Ohm speaker, halving the original load, the same 20 Volts would now produce 100 watts, and further, 200 watts into 2 Ohms. From this example we clearly see that each time the load resistance is halved, the amplifier should ideally double its output. This high current capability is especially important if the loudspeaker impedance dips into a very low range.
To make this happen, the amplifiers’ the power supply must also double its current delivery to the output transistors for this equation to hold up. The continued doubling must stop at some point, the progression cannot go on forever and, if carried too far, could end in disaster. Either the power supply will run out of current and fail to maintain the amplifiers output wattage, or worse, go beyond the capability of the output devices, creating excessive heat and eventually destruction of the transistors.
Even a speaker with a nominal 8 Ohm rating can fall below 4 Ohms at certain frequencies. If the current reserves of the amplifier are not sufficient to sustain its output wattage into low impedances, the unit will "run out of gas" sonically, at the time when the extra power is needed most. This helps to explain why a very high quality 50 watt per channel amplifier may sound less strained than another unit rated at 200 watts per channel. We also begin to understand the vast price differential among competing brands.
As we have seen, the power supply is one of the key factors in determining the current delivery capability of the amplifier. If the power supply runs out of steam, the amplifier can produce no more power, limiting the ability of the unit to deal with musical peaks and/or low impedance loads. Unfortunately, power supply components (large storage/filter capacitors, transformers etc.) are expensive, the most expensive parts within the amplifier. Consequently, we don't see inexpensive ultra-high current amplifiers. The economics of the situation simply won't allow it.
What about tubed amplifiers? Tubes, unlike transistors, are not capable of producing large amounts of current. Thus, most tube amplifiers would not be the ideal choice to power very low impedance speakers. Further, most tube amplifiers tend to be of lower power than a similarly priced solid state unit so the sensitivity of the speaker becomes another important consideration when mating a speaker with a tube amp.
But that doesn't mean tubes aren't a great alternative. Indeed, we are big fans of tube gear. Many listeners feel that better vacuum tubes designs simply convey a more natural performance than their solid state counterparts. No question that tubes have a sense of warmth and harmonic richness that can make lesser solid state gear sound a bit threadbare by comparison. Those that tout tube designs also feel the genre is particularly adept at reproducing a very believable three dimensional soundstage.
Working Together -
Ok, so now we have a basic understanding of the issues relating to the electrical match between amplifier and speaker. From here we can look at a more subjective area of the matching process, that of tonality.
Let’s say that you had chosen a loudspeaker that has a tendency toward brightness. You found that, once you got it home, your very live room exacerbated this bright character. The choice you make in mating an amplifier to your speakers will have a significant impact on whether you further aggravate or help to downplay the tonal imbalance you perceive. To best determine a match, it's important to work with a dealer that can give you a detailed description of the tonal character of the various components that fit your budget and preferably one that will let you audition the amp with your system . Ultimately, the only way to be absolutely certain of the ability of an amplifier to mate with a given speaker, is to try it.
Footnote: This explanation of how amplifiers and speakers interact is an over simplification of a very complex topic. This simple treatise is by no means intended as a complete technical explanation of the very detailed interaction that occurs between speaker and amp. I hope however, that it has served to give you a basic understanding of the important electrical relationship between the amplifier and loudspeakers.
 A simple experiment will help to better understand this statement. Using a volt ohmmeter, measure the terminals on your speaker. You will find a discrepancy between the manufacturers specification and the reading on the meter. A speaker with a nominal rating of say 8 Ohms, may measure only 6 Ohms. Why is this? Taking a reading at the speaker terminals gives you the DC resistance of the speaker, not the impedance. Measure the speaker with an impedance bridge, which makes its measurements using an AC test signal, and the speaker will produce a range of readings that vary with the frequency of the test signal.
Volts (E) = Amps (I) x Ohms (R)
Amps (I) = Volts (E) / Ohms (R)
Ohms (R) = Volts (E) / Amps (I)
R=Ohms, E=Volts, I=Amperes