If they only dim momentarily it's normal and nothing to worry about. Receivers and most tvs have capacitors in their power supplys that take a lot of juice to charge up when the unit is first turned on.
You've got L.E.S., Lazy Electron Syndrome. Many of the electrons are flabby, sitting around all day, doin' nothin. And when finally called upon to power something up, they're too weak to do the job so they've got to get help from their buds in the other appliances. What you see is a brief slump in everything else as all the electrons congregate to get the new thing up and running.
Mostly they need exercise, like playing the stereo loud for long periods of time. This gets the electrons really going!
I had that too. You just have too much junk plugged into one circuit! To solve it, I brought an extension cord - a really havy duty one, and ran my amp into a circuit on the nearest plug that was on a different circuit - in my hallway.
The alternative is to have another circuit put in. New wires would have to be run for it. Sort of dedicated.
only thing on the circuit is two 100w bulbs, a tv, and an amp. clicking the amp out of standby shouldn't bump the lights this hard. Hasn't at any other house.