Q:

Amplification of guitars

Does using more than one amplifier help increase volume to a greater extent or a little bit? I am currently using a 60 watt and 75 watt amp. I don't think the power just adds up.
What do you think about this guys? Also, using a ABC/Y switcher pedal gives three outputs and all can be activated together. Hence, we can connect to three amps. Is it feasible in relation to sound?

A:

The number attached to watts is not actually a very good guide to how loud it is. Other factors come into play, such as speaker efficiecy, sort of cabinet. Others may explain that better. However, when I use two of the same amps (for kbds, but it's the same idea) the sound seems to be more than twice. Say they're 50 watts each, for example, two makes 100 watts. Like for like, a 100 watt amp is theoretically only half as loud again as the 50 watt. So it appears that the two amps will be louder. But there again, like I said earier, there's the difference of twice as many speakers, too (two against one).

A:

No, the power ratings cannot simply be added up, in order to estimate the resulting loudness.
The power rating of the amplifier - assuming we're dealing with a consistent measure of power (more on that at the end) - is only part of the story when it comes to volume.
Sound pressure levels
Sound is essentially a wave that propagates through air. Therefore, what interests us in terms of loudness is how much the air is moving.
Sound pressure - saith Wikipedia:

is the local pressure deviation from the ambient (average, or equilibrium) atmospheric pressure, caused by a sound wave.

Sound pressure level (SPL) is:

a logarithmic measure of the effective pressure of a sound relative to a reference value.

(Source: the Wikipedia article linked to above.)
Like for like, a sound source that generates a higher SPL will be perceived as louder than one that produces a lower SPL.
It's important to note that distance is a factor. We know that something closer to us is going to be louder than something further away, but it's easy to forget to include distance in SPL comparisons.
Amplifiers and speakers
It should be clear, but bears reminding of, that whatever the amplifier may be doing, it is the speaker that creates the actual sound pressure that allows us to hear the sound.
The speaker converts the amplifier's electrical power output into motive force that creates a soundwave. The sound pressure will depend on the speaker's power rating, its efficiency and the volume of air it is able to physically shift (which, in turn, depends on its diameter). It is possible for the power ratings of the amplifier and speaker to be mismatched (albeit, this is not common these days), plus there are physical limits to what SPL an individual speaker is capable of generating. We should be aware that at no point is the entire power provided by the amplifier converted into sound - some will always be lost.
Given that the volume of air capable of being moved by a single speaker is a limiting factor in the SPL it can produce, one option is to increase the number of speakers - provided the amplifier has sufficient power to drive all of them.
More than one (combo) amplifier being driven with the same signal will be louder than a single amp, simply because more air is being moved.
Hearing sensitivity
The sensitivity of the human ear to sound varies with frequency (the linked Wikipedia article includes a chart that shows equal loudness contours by frequency). Our hearing is most sensitive around 3 kHz and - generally speaking - less sensitive to low frequencies than higher ones. This means a lower SPL will be needed for sounds at around 3 kHz to be perceived to be as loud as sounds at 100 Hz, say.
What this means, in practice, is the tone you set up will have an effect on perceived loudness. Accentuating the higher mid-range (from around 1 kHz to around 5 kHz) will make your amplifier sound louder than if the same frequency range is "scooped" out.
Phase cancellation
Whenever you have more than one speaker generating soundwaves, the waves from both sources will interact. This can have an effect on the resulting sound.
When two waves are perfectly in sync (in phase) - their peaks and troughs coincide - the amplitude of the resulting wave is increased, giving a louder sound. When they are reversed (180 degrees out of phase) - peaks from one wave meet troughs from the other - they cancel one another out (in an ideal scenario, the result would be silence; incidentally, this is how the "karaoke" function in a stereo works).
In a practical setting, the sound waves from separate speakers will not be ideal copies of one another and they will be interacting with reflected waves from the floor, ceiling and walls. This results in complex phase relationships that may lead to damping of certain frequencies (comb filtering), overall volume reduction and other sonic unpleasantness.
When using more than one amplifier, it's a good idea to position the amplifiers so that the direct signals from all speakers are in phase as much as possible.
Bottom line
Higher-rated amplifiers produce more electrical power that can be used to drive speakers, which - other things being equal - will result in greater volume than a lower-rated amplifier. However, power rating is not a measure of volume, in itself.
Using a greater number of speakers and amplifiers will give a louder sound, but there isn't a simple relationship in play, such as double the number of speakers (amplifier) means twice as loud.
The perceived volume of any combination of speakers and amplifiers might be increased by altering their position (a simple trick is to position the speaker closer to ear level, by lifting it from the floor) and equalization.
A word on power ratings
When it comes to amplifiers, marketing operates under the assumptions that more power means louder means better. For this reason, we should be wary of stated power, unless we are told how it was measured.
Maximum power measured during brief peaks ("peak power") tells us exactly nothing, other than that the seller is trying to pull a fast one. A meaningful measure of power should track the power level over many cycles, examples of such methods include average and root mean square (RMS) power output (I shall leave an investigation of the exact methodologies as an exercise for the Reader).
What is important, from a practical perspective, is that when the power ratings of different amplifiers are being compared, we should ensure that they were all measured using the same method (subject to the caveats above). The numbers given by different methods will vary greatly.

