Vowel perception is influenced by precursor sounds that are resynthesized to shift frequency regions [Ladefoged and Broadbent (1957). J. Acoust. Soc. Am. 29(1), 98-104] or filtered to emphasize narrow [Kiefte and Kluender (2008). J. Acoust. Soc. Am. 123(1), 366-376] or broad frequency regions [Watkins (1991). J. Acoust. Soc. Am. 90(6), 2942-2955]. Spectral differences between filtered precursors and vowel targets are perceptually enhanced, producing spectral contrast effects (e.g., emphasizing spectral properties of / in the precursor elicited more // responses to an /-/ vowel continuum, and vice versa). Historically, precursors have been processed by high-gain filters, resulting in prominent stable long-term spectral properties. Perceptual sensitivity to subtler but equally reliable spectral properties is unknown. Here, precursor sentences were processed by filters of variable bandwidths and different gains, then followed by vowel sounds varying from /-/. Contrast effects were widely observed, including when filters had only 100-Hz bandwidth or +5dB gain. Average filter power was a good predictor of the magnitudes of contrast effects, revealing a close linear correspondence between the prominence of a reliable spectral property and the size of shifts in perceptual responses. High sensitivity to subtle spectral regularities suggests contrast effects are not limited to high-power filters, and thus may be more pervasive in speech perception than previously thought.