600W Keyboard Amp!

A practice amp for home use is generally around 10W with a single speaker which, given the sensitivity of a typical loud speaker, would equate to around 94dB SPL at 6 feet, and would be considered as between "very loud" and "uncomfortably loud". Take that amp on stage where the audience is say 50 feet away and that SPL drops to "moderately loud" but would be drowned-out by the drums and other amplified equipment on stage. 

To re-create the same relative loudness for the audience that the player experiences in the home would require an amplifier of 300W and two loud speakers. In the same situation a 600W amp with four speakers would be "uncomfortably loud" to someone standing 50 feet away, and would be "painfully loud" to anyone within 6 feet of the amp/speaker combo. So wear ear plugs on stage if you value your hearing.

The volume control on an amp is approximately logarithmic such that reducing the volume knob from "10" to "9" reduces the amp's power by a factor of 10, that is, a 600W amp set to "9" is the equivalent to a 60W amp set to "10" and a 600W amp set to "8" is equivalent to a 60W amp set to "9"or a 6W amp set to "10"...

Therefore a 10W practice amp set to "10" is the equivalent to a 600W amp set to about "8¼".

So it really boils down to how loud you normally play your keyboard at home with a practice amp. I run mine with the volume set to about "4" giving an SPL of around 50dB (just shy of "moderately loud"). Achieving that level of loudness using a 600W amp would require setting the volume control to about "2½" ... which at first sight seems very achievable, except for two slight problems: 1) pots are approximately logarithmic and 2) they're not very accurate, especially at the low-end. So in using a powerful amp in a home environment means you don't have anything like the degree of control over the volume that you would have from using a much less powerful one.

Indeed they do, and the "headroom" issue is where my later question about what kind of amp it is was leading.