[ChrisE]

Not to complicate things, but the venue you're playing also has an effect. What usually happens is that you experiment at home and get things dialed in and then when you show up to play everything sounds completely different than it did at home.

I don't know anything about the various hz and all that, but I've generally found that if your guitar sounds harsh or brittle, try turning the mids down. And too much bass (lows) can cause feedback.