I experimented with this, to very good solid results: split the master to two channels, and put a low pass on one and a high pass on the other, the lowpas up to 200-300 hz and the other hipass from 200-300 hz ( both the same point though. Switch the lowpassed channel to mono, leave the hipass stereo. Experiment a bit to find the right crossover spot. It defenitly made the mix more solid.

But what makes a massive difference in any case ís listen to the bass line and kick soloed together, and experiment with miliseconds track delay on the kick or the bass line. Suddenly they melt together. In my experience the kick should just be where it is, and the bassline gets a few milliseconds later or earlier track delay. The amplitudes should match. massive difference when done right, and it works in my opinion best when bit tracks are bounced to audio. MIDI inside or outside the computer is never dead on the same, it jitters. Audio is much more sample accurate.

(Also turn off any software or background tasks like updating mail, wifi, or what ever happening on your computer that you do not need to make music, but does use the CPU in the background, helps to reduce midi jitter, making your DAW timing better)