Using EQ to increase loudness

Using a spectrum analyzer, find which frequencies are lacking energy, paying special attention to high mids. Use an EQ to balance energy accordingly.¹ The idea is to have the spectrum more closely resemble pink noise or an Equal loudness curve (depending on taste).

Additive EQ can have the side-effect of boosting peaks in that region, so subtractive EQ may be a better option.² Moreover, cutting early in the chain can prevent from nonlinearities reacting too much to a particular frequency range. Use EQ before saturation.

Alternatively, try Using multiband dynamics to increase loudness.

How EQ can make your music sound louder - using LESS compression and limiting - Production Advice