With modern hifi amplifiers there's no such thing like impedance mismatch, because source impedance of a standard hifi amplifier is near zero Ohm, being able to drive any loads of more than about 4Ohm (3.2Ohm according to the hifi-standards). With a standard hifi amplifier you wouldn't notice any difference. But maybe you don't have a standard hifi amplifier? Tell us what you have. Is there a transformer sitting at output of your amplifier?

There are some applications, where we need a special load impedance of loudspeaker though, to achieve high sound pressure:

1. Car sound applications, where the amplifier is only powered by 12V car battery. Then, load impedance of speaker should be rather low to obtain high sound pressure. So, you can often see, that two or more 4Ohm loudspeakers are put in parallel. This works at the cost of high load currents of course, but that's no problem with a car hifi application.

2. Battery powered applications. Here no high load currents can be drawn from battery. So, you need rather high impedant loads. This is the reason, why 16Ohm or even 32Ohm loadspeakers are fabricated.

3. There are some older amplifiers arround, which work with very early transistors. These often cannot handle very high load currents, even though rather high output voltages are no problem. To achieve a sufficient high sound pressure then 8Ohm or even 16Ohm loudspeakers should be used.

4. In PA (public address) or stage applications loudspeakers are often connected in parallel to reduce the number of needed amplifiers. Because of this, stage boxes often show 8Ohm impedance, so that at least two of them can always be paralleled.

The reason, why you observe a decrease of sound pressure in combination with a deterioration of sound quality when connecting a loudspeaker of lower load impedance could be, that you have an amplifier not providing enough current reserves. (Which I assume, because of this strange impedance of loudspeaker.) So, connecting a loudspeaker of less load impedance results in a rise of load current at same output voltage. But if there's a current limiting circuitry existant, which suddenly becomes invoked by the higher load current, then you will hear a heavily deteriorated sound with the heavily increased distortion of an invoked current limiter! Also an internal limiting of power dissipation, when using an amplifier chip, can become invoked resulting in the same.

The solution Terry suggested can alter the properties of loudspeaker so that no longer hifi performance might be obtained. The reason is, that loudspeakers are mostly designed to be driven by a low ohmic source (~0Ohm). But if you put another loudspeaker in series, the source impedance rises and especially at the resonance frequency of loudspeaker all the dampening is lost, which can heavily erode signal integrity.

Using a transformer is not really a good idea. Buy a suited loudspeaker instead.

Kai