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Most recently proposed near-end speech enhancement methods have been evaluated with the overall power (RMS) of the speech held constant. While significant intelligibility gains have been reported in various noisy conditions, an equal-RMS constraint may lead to enhancement solutions that increase the loudness of the original speech. Comparable effects might be produced simply by increasing the power of the original speech, which also leads to an increase in loudness. Here we suggest modifying the equal-RMS constraint to one of equal loudness between the original and the modified signals, based on a loudness model for time-varying sounds. Four state-of-the-art speech-in-noise intelligibility enhancement systems were evaluated under the equal-loudness constraint, using intelligibility tests with normal-hearing listeners. Results were compared with those obtained under the equal-RMS constraint. The methods based on spectral shaping and dynamic range compression yielded significant intelligibility gains regardless of the constraint, while for the method without dynamic range compression the intelligibility gain was lower under the equal-loudness than under the equal-RMS constraint.