Good explanation by lindsayt. To answer gasolin in brief
, in 4 ohm loads more current is drawn and the PSU becomes the limitation.
Let's say you are playing a 1kHz test tone. The amp tries to maintain a constant voltage assigned by you via the volume controls of say 20V. At 8 ohms, the current draw will be 2.5A and speakers will draw power of 50W (
Ohm's Law). 20V/8Ω=2.5A
Now if you attach a 4 ohm speaker, to maintain the same loudness of 20V, the current must increase to 5A. 20V/4Ω=5A. The system wont be louder in that 4 ohms speaker. It will use twice the current to remain as loud as the 8 ohms speaker and thus draw twice the power - 100W. (P=VA, 100W=20V*5A)
The problem begins if the power supply can't provide those 5A because the bridge rectifier, caps and transformer are not optimized for such current draw. The capacitors are limited to a certain amount of energy (joules) they can hold and the diodes and transformer are limited at the speed they can recharge them as joules get sucked by the speakers in lower impedances. In such scenario of say only 4.23A delivered by the PSU, by Ohm's Law the voltage must go down since the speaker impedance (resistance) is constant. 4.23A*4Ω=16.92V. When voltage sags, this is the amplifier clipping. Most amplifiers these days can survive 4 ohms loads to a certain point of loudness demand, but if the speaker dip lower at certain frequencies, say in 2 ohms, things get very bad for the amp.
For this example I used a 1kHz test tone. When music is playing, the voltage is constantly changing but that still means Ohm's Law applies at all points in time. This can get a bit confusing when I say amplifier's job is to maintain constant voltage. This part was a bit of a head scratcher for me to understand when I was reading up on how amplifiers work. I find it easier to explain using a steady single frequency test tone.
As music changes with quieter and louder parts, different amounts of voltage and current are going from the amplifier to the speakers. And the speakers also have continuously changing impedance depending on frequency as music is playing. Certain music requests gobs of current only for a brief period of time (transient peak), so the PSU has time to recharge the caps before the next transient comes and draws more current. But with music like EDM and some hiphop tracks the long bass passages draw a lot of current for longer periods than for example jazz. If the PSU doesn't have lots of caps and big diodes following a big transformer to charge them up, the voltage will sag, amplifier will clip. And when amplifiers clip, the speaker vice coils don't go their usual fast back and forward in the magnet gap which makes them cool efficiently. During clipping longer periods of time are spent stuck in a position at one end of the magnet gap and this accumulates heat, which burns the hair thin voice coils, especially on tweeters.
Which leads us to the conclusion that small amps are bigger speaker killers than large amps. More clipping = more burned voice coils.