Speakers appear to adopt strategies to improve speech intelligibility for interlocutors in adverse acoustic conditions. Generated speech, whether synthetic, recorded or live, may also benefit from context-sensitive modifications in challenging situations. The current study measured the effect on intelligibility of six spectral and temporal modifications operating under global constraints of constant input-output energy and duration. Reallocation of energy from mid-frequency regions with high local SNR produced the largest intelligibility benefits, while other approaches such as pause insertion or maintenance of a constant segmental SNR actually led to a deterioration in intelligibility. Listener scores correlated only moderately well with recent objective intelligibility estimators, suggesting that further development of intelligibility models is required to improve predictions for modified speech.