How to clean inside Amps and PreAmps

Actually the Bosch came up with the laundry machine especially made for washing the Amps/Preamps.
They still working on a dryer

Dust-off or spray electronic cleaner. 

 I am not going to weigh so much on the dust issue, as it Can be worse in some areas of the country than others. With some 'common sense' you should be able to approach your situation appropriately.
 Like others here, I have a short story that may be worth noting. I took on a repair of a piece of equipment a few years ago, that startled me when I took off the cover. I could not  believe what I saw. Someone, recently before the unit was shipped to me, had sprayed every known corner and everywhere in between with something like De-Oxit on the inside of this unit! It was WET with the stuff. In the end, I was able to Q-tip some out, use electronic spray cleaner, and just about anything else to rid the circuit boards, transformer, etc of the substance. I had nothing to lose in this case, and was quite surprised that the unit actually worked after the procedure with no known troubles after I sent it back to my customer. So, when the idea comes along that a product is supposed to be used as 'an electronic protectant' , don't take it too far. I would add that if a product is NOT made specifically for the task at hand, use at your own risk.

I use an air brush compressor where I can direct fine jets of air ad infinitum with a gentle enough amount of pressure regularly to avoid substantial amounts of dust accumulation.

Don’t use compressed air around moving parts like rotary controls etc as the compressed air can drive dust particles deep inside the unit.  You can use a low pressure anti-static vacuum brush to evacuate the dust.  These are commonly used in the computer maintenance industry