This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
Almost any amplifier can drive almost any load if you don't turn
the volume up too high. Tube amplifiers are one exception.
Some amps clip if you play them too loud. This is bad and
damages speakers. Other amplifiers shutdown if they are asked
to play too loud. Many will overheat, with bad consequences.
However, in almost all cases, it takes seriously loud sound or
low speaker resistance (less than 4 ohms) to do damage. Running
two sets of 8 ohm speakers at once with common amplifiers
represents a 4 ohm load. Four sets of 8 ohm speakers makes a 2
ohm load. Two sets of 4 ohm speakers also makes a 2 ohm load.
If you stay sober and don't turn it up past the point where it
distorts, you are PROBABLY safe with most amplifiers and most
loads. See 11.3 for more information.
You can raise the impedance of a speaker by a few different
methods. However, each has drawbacks. If your amplifier won't
drive your speakers, AND you are sure that the problem is that
the speakers are too low impedance, you might try one of these
A) Add a 4 ohm resistor in series with the speaker.
This requires a high power resistor, because the
resistor will dissipate as much power as the speaker.
Doing this will almost always hurt sound quality, too.
This is caused, in part, by the fact that speakers do
not have constant resistance with frequency. See 11.3
for more information on this.
B) Use a matching transformer. There are speaker matching
transformers which can change from 4 ohm to 8 ohm, but
a high quality transformer like this can cost as much
as a common receiver. Also, even the best transformer
will add some slight frequency response and dynamic
C) Use two identical speakers in series. If you have two
4 ohm speakers which are the same make and model, you
can wire them in series and make an equivalent speaker
with 8 ohm impedance. The sound from that "new speaker"
will not be as precisely localized as it would from one
speaker, so your stereo image may be hurt. Also, it
requires that you buy twice as many speakers as you
might have bought otherwise. However, this technique
has one side benefit. Two speakers can handle twice the
power of one.