How to clean inside Amps and PreAmps

First, I would put the covers back on. I doubt that you are increasing performance by having them off but you are increasing potential damage to your unit and possibly creating a fire hazard.

After you return the lid to its proper place the only entry for dust would be air vents. When not using your pre or amp you can drop a cloth over them to minimize dust settling in. If you do this you will not have an issue with dust.

To remove the dust that is currently there I found that a loose, natural hair painters brush works best.

Thanks for you suggestion. That's what I have been doing. I just removed the cover 2 days back and found dust inside. I always covered them with cloth after usage. thanks for Painters Brush suggestion.

BTW, stay away from compressed air as it can create moisture and freeze components.

I would put the covers back on as well. Then cover as you have been. Why did you remove the covers? If they are SS you are not doing yourself any favors. Sometimes the lids are integral to the heat sink operation and need to be present. Be careful and what is microdust? Can you see it with the naked eye?