What is the difference between Watt and Volume?

People say that if the amp has 100W more power, the sound will be 3dB louder. But with the volume on the pre-amp, you can make the sound as loud as you wish. What is the difference?

You suffer from several delusions here.

First off, when you DOUBLE the power to your speaker, you can increase the apparant volume by a barely discernable 3 decibles. Generally, this will only be called into play on peaks, not the overall listening level. While your "100 more watts" example might hold true for a 100 watt amplifier, it will not hold water when starting with amps with other power ratings.

Second, the volume control only limits the maximum volume at which the speaker will play, not add to it. The maximum is determined by the watts the amp will put out (That, and speakers sensitivity but that's another topic) . If you ain't got that 100 watts in the first place, it will never make it to the speaker at all. Likewise for double the power... or half the power for that matter.

Look at it like horsepower in a car and the gas pedal. If the power ain't there, you can press on that pedal all day and you won't go any faster.

Watt is power. Volume is decibels. It takes double the wattage to incease speaker volume by 3 dbs. If we assume a speaker has a sensitivity of 87dbs at 1 watt at one meter the chart would go something like this:

W - db
1 - 87
2 - 90
4 - 93
8 - 96
16 - 99
32 - 102
64 - 105
128 - 108

As you can see, the difference between a 65 WPC amp and a 100 WPC amp is relatively minor but people think it is major because 100 sounds much better than 65.

There are a few things to keep in mind here. The vast majority of your listening is done well below 1 watt. 1 watt is VERY loud. How loud? that depends on your speaker's sensitivity, but even on relatively piggy speakers, it's loud. (don't take the word "piggy" as bad. Designing a woofer to go deep accurately means that you sacrifice sensitivity or loudness per watt. Life is full of tradeoffs).

Now, back on the subject of watts. A watt rating on one brand of receivers isn't going to equate very well with a watt rating on another, especially if you are going from an entry level line (Sony, Pioneer, JVC, Panasonic/Technics, Kenwood, etc) to a more upscale line (Yamaha, Denon, Marantz, h/k, etc). The entry level products are designed to look good on the spec sheet to get uneducated consumers to think they are getting a deal. The upscale brands are designed to more effectively deal with real world speaker loads. In other words, a lower rated upscale receiver will typically play louder, longer than an entry level receiver with a much higher watt rating.

Notice I didn't use the word "power". Power is the ability to do work, an audio "watt rating" is the ability to pass a very basic test that isn't based on reality. I don't feel it is appropriate to confuse the two concepts.

So, without knowing it, you asked a question with about 100 million possible answers. BTW, a preamp will not add any power to an amplifier. You get what the amplifier can do into the load presented by the speaker you choose. What the potential maximum output will be changes by the millisecond. Sometimes it can be pretty high, sometimes it will be very very low. So given the choice between more and better, choose better. You will be glad you did.