I seek out new bands through AllMusic recommendations, Last.Fm recommendations, music magazines, blogs, record labels, friends, compilations, and soundtracks.
I don't find AMG or last.fm to slow down the rate you discover new great bands at all. That doesn't make any sense. If anything, it greatly helps. I can read about a great band in Filter but they might not be the genre of music I'm into. Whereas if AllMusic says that this artist has released a 4.5 star album and they sound like Belle And Sebastian, Cub, and the Magnetic Fields, I'll be pretty damn sure that I'm going to love them.