The amplifier powering the subwoofer draws far more current than most amplifiers used to power high/mid range speakers. The subwoofer/sub amp draw is what causes issues for the electrical system. I haven't seen a 2/4 channel amp that draws enough to dim lights yet, mind you I haven't actually seen too many setups. I guess if your alternator was tiny, then any extra load (high/mid range powered speakers) could cause dimming as well. Usually its due to the subwoofer/sub amplifier.

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When I say dimming lights, I mean adding strain to the electrical system, therefore possibly damaging your alternator. Figured I would set that straight before I get criticized lol.

 

My personal experience with dimming lights is this; I have a newer car with a 190 amp H.O. alternator, big 3 with 0 awg all around, heavy duty 3/4" ground bolt through the frame and with a 0 awg return to the battery, 1000cca heavy duty Interstate battery,...and I still get minimal dimming under high output situations, even on the highway at 140k. I've given up on trying to get rid of it completely, it's too much money to spend on something that I don't really need that often...I like my hearing. Anyway, my point is you may not be able to get rid of it completely, but you can improve it drastically. Prior to doing the electrical upgrades, the lights would dim to 1/2 intensity on heavy bass notes(tocata in d minor), now it's a slight flicker. Keep in mind it cost close to 600.00 for this upgrade, and I did the work myself.

 

A HO alternator would be nice... but like you said, that costs money. I should do the big 3... Everything takes time and money, both I'm short on. I only chat on this forum from work lol.